the prevalence of psychiatric disorders among people with intellectual disabilities: an analysis of...

The Prevalence of Psychiatric Disordersamong People with Intellectual Disabilities:An Analysis of the LiteratureSimon Whitaker and Stephen Read

The Learning Disability Research Unit, The University of Huddersfield, Queensgate, Huddersfield, UK

Accepted for publication 23 September 2005

Background It has often been stated that the prevalence

of psychiatric disorders in people with intellectual dis-

abilities is greater than it is in the population as a

whole.

Method The epidemiological studies on psychiatric disor-

ders in people with intellectual disabilities were

reviewed.

Results There is evidence that the prevalence of psychi-

atric disorder is greater in children with intellectual dis-

abilities, compared with children with normal IQs, that

it is higher in both adults and children with severe intel-

lectual disabilities compared with people with mild or

no intellectual disability and that the rate of problematic

behaviour is higher in both adults and children having

intellectual disabilities, than in their non-disabled peers.

Conclusions There is no sound evidence that the preval-

ence of psychiatric disorders in adults with mild intel-

lectual disability is greater than in the population as a

whole.

Keywords: learning disability, mental illness, mental

retardation, prevalence

Introduction

It has frequently been stated that the prevalence of

psychiatric disorder among people with intellectual dis-

abilities is higher than it is in the population as a whole

(Campbell & Malone 1991; Menolascino & Fleisher 1991;

Vitiello & Behar 1992; Borthwick-Duffy 1994; Demb et al.

1994; Berry & Gaedt 1995; Bongiorno 1996; Ballinger

1997; Jopp & Keys 2001; Moss 2001; Chaplin 2004). If

this is the case, it suggests that having an intellectual

disability predisposes a person to developing a psychi-

atric disorder.

It is the aim of this paper to review the epidemiologi-

cal studies on psychiatric disorder in people with intel-

lectual disabilities, in order to ascertain if there is

evidence that the prevalence of psychiatric disorder is

higher in the population with intellectual disability and

if this applies to people with intellectual disabilities as a

whole or just to particular sub-groups.

In order to conclude that the rate of psychiatric disor-

der is higher among people with intellectual disabilities,

the epidemiological studies must be comparable with

those used to determine prevalence in the population as

a whole. That is, they should use the same methodo-

logy, definitions of psychiatric disorders and diagnostic

criteria as these studies and equivalent sampling meth-

ods or an appropriately matched control group. It is

therefore necessary to look at the epidemiological stud-

ies of psychiatric disorder in the population as a whole

in order to compare these studies with those on people

with an intellectual disability, and to consider what an

appropriate sample of people with learning disabilities

would be.

Bland and colleagues carried out one of the most com-

prehensive epidemiological studies on the prevalence of

psychiatric disorders in adults, in a whole population,

in Edmonton, Canada. They report a lifetime overall

prevalence of 33.8% (Bland et al. 1988a) and a 6-month

prevalence of 17.1% (Bland et al. 1988b). Other studies

include those by Lee (1998), who reported a lifetime pre-

valence of 40% in Korea, and Myers et al. (1984) who

looked at the 6-month prevalence rate in the cities of

New Haven, Baltimore and St Louis in the US and

found the overall rate of mental illness to be 18.1%.

Anderson et al. (1987) carried out an epidemiological

study of 11-year-old children in New Zealand and

Journal of Applied Research in Intellectual Disabilities 2006, 19, 330–345

� 2006 BILD Publications 10.1111/j.1468-3148.2005.00293.x

found a 1-year prevalence for one or more disorders of

17.6%. Bird et al. (1988) looked at the presence of psychi-

atric disorders in 843 children in Puerto Rico aged 4–

16 years and found 17.9% to have a DSM-III diagnosis.

Costello et al. (1996) completed a large epidemiological

study of psychiatric disorders in children aged 9–

13 years in North Carolina. The 3-month prevalence of

any DSM-III-R axis 1 disorder was 20.3%.

A key factor in these studies is that rates of psychi-

atric disorders are based on a random sampling of the

population as a whole. It is therefore important to estab-

lish whether the studies carried out on people with

intellectual disabilities also used random samples. Whi-

taker (2004) has suggested that this may not be the case,

with many of the samples of people with intellectual

disabilities used in epidemiological studies. He has

shown that there is a major disparity between the num-

ber of people who, if assessed, would be regarded as

having an intellectual disability and the number of peo-

ple known to services to have an intellectual disability.

Theoretically, if intellectual disability is defined purely

in terms of having an IQ below 70 during the develop-

mental period then one would expect that 3% of the

population would have an intellectual disability. If an

additional criterion, of having additional deficits in

adaptive functioning, such as is adopted in the DSM-IV

criteria for having a mental retardation (American Psy-

chiatric Association 2000) then one would expect there

to be about 1% of the population to have an intellectual

disability. However, the proportion of the population

who are known to services as having an intellectual dis-

ability is about 0.25%. This is what Kushlick (1975) has

referred to as ‘administrative prevalence’. As having a

mental disorder would be likely to draw a client to the

attention of services and so result in their intellectual

disability being identified, it is likely that a sample of

people with intellectual disabilities who were known to

services would have a higher rate of psychiatric disor-

der. In the light of this possibility, this review focuses

particularly on the extent to which the samples are rep-

resentative of people with intellectual disabilities as a

whole.

Identification of epidemiological studies

The literature was searched for epidemiological studies

on the prevalence of psychiatric disorder in people with

intellectual disabilities. This was done by following up

the reference cited in the papers that made the claim that

the rate of psychiatric disorder was higher among people

with intellectual disabilities, and doing a search of ‘Web

of Science’, ‘MEDLINE’, ‘PubMed’ and ‘PsychINFO’

using the following key words ‘Mental Illness’, ‘Psychi-

atric Condition’, ‘Depression’, ‘Personality Disorder’,

‘Schizophrenia’, ‘Mental Retardation’, ‘Learning Disabil-

ity’, ‘Mental Handicap’ and ‘Mental Deficiency’. Studies

cited in the obtained papers were then followed up. This

process continued until no more new studies were iden-

tified. Studies were included if they looked at the preval-

ence of psychiatric disorder (or an equivalent term such

as psychiatric condition or mental illness) in a commu-

nity population of people with intellectual disabilities.

Studies were not included if the sample was drawn from

a single setting which would only represent a limited

range of intellectual disabilities or could reasonably be

expected to have a disproportionate number of people

with a psychiatric disorder. Examples of such samples

include patients referred to an outpatient clinic (Philips

& Williams 1975; Reid 1980; Benson 1985; Myers

1987a,b), the population of a hospital (Heaton-Ward

1977; Wright 1982; Day 1985; Reid & Ballinger 1987;

Matson et al. 1988; Glue 1989; Vitiello et al. 1989), and a

day service (Ballinger & Reid 1977; Reiss 1990). Studies

were also excluded if they did not include people with

mild intellectual disabilities (Patel et al. 1993; Moss &

Patel 1997).

The identified studies that report exclusively on chil-

dren are presented in Table 1, together with information

on the nature of the sample, the overall rate of psychi-

atric disorder and the prevalence of the individual diag-

nostic conditions reported and/or symptoms. Other

studies that dealt with adults only or adults and chil-

dren are presented in Table 2. The terminology with

regard to the overall rates of psychiatric disorders, indi-

vidual conditions and symptoms reported on in both

tables is that used in the cited paper.

The Studies

Studies of children

The studies that looked at the prevalence of psychiatric

disorder in children with an intellectual disability are

presented in Table 1. There is some variation in the cri-

teria used to describe an intellectual disability. Only

one study (Birch et al. 1970) used a dual criterion of

having both a low IQ and additional social and educa-

tional problems. Most studies defined their sample on

IQ alone and the IQ figures used varied between stud-

ies. In addition, three studies (Linna et al. 1999; Emer-

son 2003 and Molteno et al. 2001) used pragmatic

definitions where inclusion is determined by factors

Journal of Applied Research in Intellectual Disabilities 331

� 2006 BILD Publications, Journal of Applied Research in Intellectual Disabilities, 19, 330–345

Tab

le1

Stu

die

so

fth

ep

rev

alen

ceo

fp

sych

iatr

icd

iso

rder

sin

chil

dre

nw

ith

anin

tell

ectu

ald

isab

ilit

y

Stu

dyD

escr

ipti

onof

sam

ple

How

diag

nos

isw

asob

tain

edR

ates

ofps

ychi

atri

cdi

sord

erre

port

ed

Bir

chet

al.

(197

0)A

llch

ild

ren

aged

8–10

yea

rsw

ere

surv

eyed

in19

62in

Ab

erd

een

.10

4m

eta

crit

eria

of

men

tal

sub

no

rmal

ity

of

hav

ing

anIQ

<75

and

add

itio

nal

soci

alan

d

edu

cati

on

alp

rob

lem

s.T

his

was

1.26

%o

f

the

po

pu

lati

on

of

8–10

-yea

ro

lds

Inte

rvie

ws

wer

eco

nd

uct

edw

ith

teac

her

san

d/

or

care

rsan

dif

po

ssib

le

the

chil

d,

by

ach

ild

psy

chia

tris

t

An

ov

eral

lra

teo

f30

%h

ada

clea

r

psy

chia

tric

abn

orm

alit

y20

%a

po

ssib

le

abn

orm

alit

y

Dek

ker

etal

.(2

002)

Asa

mp

leo

f96

8ch

ild

ren

age

6–18

yea

rs

wh

oat

ten

ded

sch

oo

lsfo

rch

ild

ren

wit

h

inte

llec

tual

dis

abil

itie

sin

anar

eao

fH

ol-

lan

d.

Ab

ou

t2%

of

Du

tch

chil

dre

nat

ten

-

ded

such

sch

oo

lsan

dh

adIQ

<80

Th

eC

hil

dB

ehav

ior

Ch

eck

list

was

com

ple

ted

by

par

ents

or

mai

nca

rer

and

the

Tea

cher

’sR

epo

rtF

orm

was

com

ple

ted

by

teac

her

s

Ov

eral

lab

ou

t50

%w

ere

fou

nd

inth

e

bo

rder

lin

eo

rcl

inic

alra

ng

e.M

ean

of

bo

th

scal

esfo

rin

div

idu

alco

nd

itio

ns:

wit

hd

raw

al,

14.7

%;

som

atic

com

pla

ints

,

9.8%

;an

xie

ty/

dep

ress

ion

,13

.7%

;so

cial

pro

ble

ms,

28.6

%;

tho

ug

ht

pro

ble

ms,

12.8

%;

atte

nti

on

pro

ble

ms,

24.0

%;

del

inq

uen

tb

ehav

iou

r,13

.7%

;ag

gre

ssiv

e

beh

avio

urs

,19

.2%

;in

tern

aliz

ing

,36

.7%

;

exte

rnal

izin

g,

38.5

%

Dek

ker

&K

oo

t(2

003)

Asa

mp

leo

f47

4ch

ild

ren

age

7–20

yea

rs

wh

oat

ten

ded

sch

oo

lsfo

rch

ild

ren

wit

h

inte

llec

tual

dis

abil

itie

sin

anar

eao

f

Ho

llan

d.

Ab

ou

t2%

of

Du

tch

chil

dre

n

atte

nd

edsu

chsc

ho

ols

and

had

IQ<

80

Th

eD

iag

no

stic

Inte

rvie

wS

ched

ule

(DIS

C-I

V-P

)w

asg

iven

top

aren

tsb

y

trai

ned

inte

rvie

wer

s

An

yD

SM

-IV

dia

gn

osi

s,38

.6%

;an

yan

xie

ty

dis

ord

er,

21.9

%(s

pec

ific

ph

ob

ia,

17.5

%;

soci

alp

ho

bia

,2.

5%;

sep

arat

ion

anx

iety

dis

ord

er,

2.1%

).A

ny

mo

od

dis

ord

er,

4.4%

(maj

or

dep

ress

ive

dis

ord

er,

1.7%

;

dy

sth

ym

icd

iso

rder

,2.

3%).

An

y

dis

rup

tiv

ed

iso

rder

25.1

%(A

DH

D,

14.8

%;

op

po

siti

on

ald

efian

td

iso

rder

,13

.9%

)

Ein

feld

&T

on

ge

(199

6a,b

)Q

ues

tio

nn

aire

sw

ere

com

ple

ted

by

454

par

ents

or

care

rso

nch

ild

ren

age

4–

18y

ears

liv

ing

infi

ve

reg

ion

so

fN

ew

So

uth

Wal

esA

ust

rali

a.T

he

sam

ple

incl

u-

ded

all

the

peo

ple

wit

hm

od

erat

eto

pro

-

fou

nd

inte

llec

tual

dis

abil

itie

sin

fou

ro

f

thes

ere

gio

ns

and

ara

nd

om

sam

ple

in

the

fift

h.

Th

eyal

sofo

un

dal

lth

ep

eop

le

wit

hm

ild

inte

llec

tual

dis

abil

ity

(IQ

<71

)

kn

ow

nto

serv

ices

,h

ow

ever

,n

ote

that

they

wer

ew

ell

sho

rto

ffi

nd

ing

them

all,

mil

din

tell

ectu

ald

isab

ilit

ym

ade

up

on

ly

30.4

%o

fp

arti

cip

ants

Par

ents

or

oth

erca

rers

com

ple

ted

the

Dev

elo

pm

enta

lB

ehav

iou

rC

hec

kli

st

(DB

C),

inm

ost

case

sth

isw

asm

aile

d

toth

em

40.7

%h

ada

defi

nit

ep

sych

iatr

icd

iso

rder

or

maj

or

emo

tio

nal

/b

ehav

iou

ral

dis

turb

ance

332 Journal of Applied Research in Intellectual Disabilities


Tab

le1

Co

nti

nu

ed

Stu

dyD

escr

ipti

onof

sam

ple

How

diag

nos

isw

asob

tain

edR

ates

ofps

ychi

atri

cdi

sord

erre

port

ed

Em

erso

n(2

003)

264

chil

dre

nb

etw

een

5an

d15

yea

rsw

ho

had

bee

nid

enti

fied

asa

hav

ing

inte

llec

tual

dis

abil

itie

s,b

ased

on

ap

rag

mat

icd

efin

itio

n

of

inte

llec

tual

dis

abil

ity

,fr

om

ast

rati

fied

ran

do

msa

mp

leo

f10

438

chil

dre

nfr

om

En

gla

nd

,S

cotl

and

and

Wal

es,

com

pri

sin

g

2.53

%o

fth

esa

mp

le

Str

uct

ure

din

terv

iew

so

fth

em

ain

care

and

ap

ost

alq

ues

tio

nn

aire

giv

ento

teac

her

s

Ov

eral

l39

.0%

had

aIC

D-1

0d

iag

no

sis.

An

xie

tyd

iso

rder

,8.

7%;

dep

ress

ion

,1.

5%;

con

du

ctd

iso

rder

,25

.0%

;h

yp

erk

ines

is,

8.7%

;P

DD

,7.

6%;

Tic

dis

ord

er,

0.8%

;ea

t-

ing

dis

ord

er,

0.4%

;p

sych

osi

s,0.

0%

Gil

lber

get

al.

(198

6)16

4ch

ild

ren

aged

13–1

7y

ears

wh

oh

adb

een

iden

tifi

edas

hav

ing

men

tal

reta

rdat

ion

,in

Go

then

bu

rgS

wed

en,

fro

ma

tota

lp

op

ula

tio

n

of

2449

8(o

r0.

67%

of

the

po

pu

lati

on

).T

hey

defi

ned

men

tal

reta

rdat

ion

ash

avin

gan

IQ

of

<70

Th

ech

ild

ren

wer

eex

amin

edb

ya

do

cto

r,

thei

rca

sen

ote

sex

amin

edan

da

par

ent

was

inte

rvie

wed

An

ov

eral

l54

.3%

had

ap

sych

iatr

ic

dis

ord

er:

dep

ress

ion

,2.

4%;

emo

tio

nal

dis

ord

er,

6.7%

;co

nd

uct

dis

ord

er,

7.9%

;

psy

cho

tic

beh

avio

ur,

27.4

%(s

chiz

op

hre

-

nia

,1.

2%;

Au

tism

,4.

9%;

tria

do

fla

ng

uag

e

and

soci

alim

pai

rmen

t,8.

5%;

sev

ere

soci

al

imp

airm

ent,

1.2%

;A

sper

ger

’s,

0.6%

),

oth

erd

iag

no

sis,

2%.

Lin

na

etal

.(1

999)

90ch

ild

ren

age

8y

ears

wh

oat

ten

ded

sch

oo

ls

for

the

edu

cati

on

ally

sub

no

rmal

or

trai

nin

g

sch

oo

lsin

Fin

lan

d,

com

pri

sin

g1.

5%o

fa

sam

ple

of

5804

8-y

ear-

old

chil

dre

n

Qu

esti

on

nai

res

wer

eco

mp

lete

db

yp

aren

ts

and

teac

her

s.In

add

itio

n,

the

Ch

ild

Dep

ress

ion

Inv

ento

ryw

asco

mp

lete

dfo

r

each

chil

d

50%

of

the

inte

llec

tual

dis

abil

itie

sch

ild

ren

had

po

ssib

lep

sych

iatr

icd

iso

rder

so

nat

leas

to

ne

qu

esti

on

nai

re.

11%

sco

rein

the

dep

ress

ion

ran

ge.

On

par

ents

rati

ng

:

emo

tio

nal

dis

turb

ance

19.5

%,

beh

avio

ura

l

dis

turb

ance

6.9%

mix

edem

oti

on

al/

beh

avio

ura

l5.

9%;

on

teac

her

sra

tin

gs

emo

tio

nal

dis

turb

ance

9.0%

,b

ehav

iou

ral

dis

turb

ance

22.5

%,

mix

edb

ehav

iou

ral

emo

tio

nal

3.4%

Mo

lten

oet

al.

(200

1)35

5ch

ild

ren

bet

wee

n6

and

18y

ears

atte

nd

ing

two

spec

ial

sch

oo

lso

ra

trai

nin

gce

ntr

ein

Cap

eT

ow

nS

ou

thA

fric

a.It

isn

ot

clea

rh

ow

inte

llec

tual

dis

abil

ity

was

defi

ned

or

the

per

cen

tag

eo

fch

ild

ren

wh

oat

ten

ded

spec

ial

sch

oo

lb

ut

on

ly36

%o

fth

esa

mp

leh

adm

ild

inte

llec

tual

dis

abil

ity

,su

gg

esti

ng

the

sam

ple

was

bia

sed

toth

em

ore

sev

ere

deg

ree

of

inte

llec

tual

dis

abil

ity

Th

eD

evel

op

men

tal

Beh

avio

ura

l

Ch

eck

list

-Tea

cher

ver

sio

nw

asco

mp

lete

d

by

teac

her

s

31%

of

the

chil

dre

nh

ada

tota

lsc

ore

of

30

or

mo

rean

dso

wer

eco

nsi

der

edto

hav

e

ad

ual

dia

gn

osi

s

Ru

tter

etal

.(1

970)

59ch

ild

ren

aged

9–10

yea

rsw

ho

inte

llec

tual

reta

rdat

ion

wh

osc

ore

d2

stan

dar

dd

evia

tio

ns

bel

ow

the

mea

no

f23

34ch

ild

ren

,li

vin

gin

the

Isle

of

Wig

ht,

on

the

sho

rtfo

rmo

fth

e

WIS

C.

Th

eym

ade

up

2.5%

of

the

chil

dre

n

Qu

esti

on

nai

res

com

ple

ted

by

par

ents

and

teac

her

san

dan

ind

ivid

ual

asse

ssm

ent

by

ap

sych

iatr

ist

23.6

%sh

ow

eda

mar

ked

dis

ord

er



such as the type of education received rather than an

IQ score.

Two of the studies (Birch et al. 1970; and Rutter et al.

1970) examine all the children with intellectual disabilit-

ies at a specific age, living in a specified location, so

sampling as such is not an issue. Emerson (2003) also

used a sample of children that made up 2.5% of a strati-

fied random sample of British children, although as

noted above, he used a pragmatic definition of intellec-

tual disability so it was not entirely clear how represen-

tative the sample was of children who would meet

more formal criteria. Of the other studies, it is not clear

how representative the samples are of children with

intellectual disabilities as a whole. Linna et al. (1999) use

a sample that represents 1.5% of the population and so

could be representative. Although Dekker et al. (2002)

and Dekker & Koot (2003) use samples that represent

about 2% of Dutch schoolchildren, the IQ figure they

use is 80 rather than 70 so one would expect it to be

about 9% of the population. Others, notably Einfeld &

Tonge (1996a,b) and Molteno et al. (2001), used samples

where the proportion of children with mild intellectual

disability is considerably lower than one would expect,

had the sample been representative of children with

intellectual disabilities as whole.

With regard to the type of prevalence reported, only

Dekker et al. (2002) and Dekker & Koot (2003) are expli-

cit about this – Dekker et al. (2002) reporting a 6-month

prevalence and Dekker & Koot (2003) a 1-year preva-

lence. In the other studies it is not clear, though the

indications are that it was point prevalence.

There are therefore at least three studies that appear

to have good representative samples of all intellectual

disabilities in Birch et al. (1970); Emerson (2003) and

Rutter et al. (1970). These studies all show rates of psy-

chiatric disorder that are consistently higher than those

shown in the epidemiological studies of children with-

out intellectual disabilities. Anderson et al. (1987) reports

a 1-year prevalence of 17.6%, Bird et al. (1988) a rate of

17.9% and Costello et al. (1996) 20.3%. Moreover, Rutter

et al. (1970) found the rate of psychiatric disorder in the

children without an intellectual disability to be 1.4%,

significantly less than the 23.6% for those with intellec-

tual disabilities.

The overall rate of psychiatric disorder reported in

the other studies in Table 1 is similar to that reported

by Birch et al. (1970), Emerson (2003) and Rutter et al.

(2003) even though the samples they use are questiona-

bly representative of people with intellectual disabilities

as a whole. Overall, the studies therefore provide evi-

dence that the rate of psychiatric disorder in childrenTab

le1

Co

nti

nu

ed

Stu

dyD

escr

ipti

onof

sam

ple

How

diag

nos

isw

asob

tain

edR

ates

ofps

ychi

atri

cdi

sord

erre

port

ed

Str

om

me

&D

iset

h(2

000)

178

chil

dre

nag

ed8–

13y

ears

fro

man

ori

gin

al

sam

ple

of

182

chil

dre

nw

ho

wer

eid

enti

fied

ash

avin

gm

enta

lre

tard

atio

n(I

Q<¼

70),

in

aN

orw

egia

nco

un

ty,

ap

rev

alen

ceo

f0.

62%

of

the

po

pu

lati

on

of

8–13

-yea

ro

lds

inth

e

cou

nty

Ex

amin

atio

nb

ya

do

cto

ran

d

con

firm

atio

nb

ya

chil

dp

sych

iatr

ist

36.5

%h

ada

ICD

-10

dia

gn

osi

s:h

yp

erk

ines

ia,

16%

;p

erv

asiv

ed

evel

op

men

tal

dis

ord

ers,

8%

(au

tism

,4.

5%;

Ret

ts,

0.5%

;A

sper

ger

’s,

0.5%

;

un

spec

ified

,2.

5%),

beh

avio

ura

lo

rem

oti

on

al

con

dit

ion

s,6%

(en

cop

resi

s/en

ure

sis,

0.5%

;

ster

eoty

py

,5.

5%),

con

du

ct,

3%,

An

xie

ty/

ph

ob

ias/

ob

sess

inal

-co

mp

uls

ive,

3%,

Tic

s,1%

AD

HD

,at

ten

tio

nd

efici

th

yp

erac

tiv

ity

dis

ord

er.



Tab

le2

Stu

die

so

fth

ep

rev

alen

ceo

fp

sych

iatr

icd

iso

rder

sin

adu

lts

on

lyo

rad

ult

san

dch

ild

ren

wit

han

inte

llec

tual

dis

abil

ity

Stu

dyD

escr

ipti

onof

sam

ple

How

diag

nos

isw

asob

tain

edR

ates

ofps

ychi

atri

cdi

sord

erre

port

ed

Bo

rth

wic

k-D

uff

y&

Ey

man

(199

0)

7860

3p

eop

lew

ith

MR

aged

0–86

yea

rsw

ho

wer

ere

ceiv

ing

a

serv

ice

fro

mth

eC

alif

orn

ia

Dep

artm

ent

of

Dev

elo

pm

enta

l

Ser

vic

esin

1986

.T

he

sam

ple

con

sist

edo

f0.

23%

of

the

po

pu

lati

on

of

Cal

ifo

rnia

asa

wh

ole

(bas

edo

n20

00ce

nsu

s).

Defi

nin

go

f

MR

was

inac

cord

ance

wit

hth

e

AA

MR

defi

nit

ion

Rec

ord

sw

ere

exam

ined

.D

ata

wer

e

on

lyu

sed

ifth

ed

iag

no

sis

had

bee

nm

ade

by

aq

ual

ified

pro

fes-

sio

nal

10.0

%h

add

ual

dia

gn

osi

s

Co

op

er(1

997)

134

eld

erly

(E)

adu

lts

(ag

e

65+

yea

rs)

and

73y

ou

ng

(Y)

adu

lts

(ag

e20

–64

yea

rs)

fro

mth

e

Lei

cest

ersh

ire

LD

reg

iste

r.T

her

e

wer

e14

3p

eop

leag

ed65

+y

ears

on

the

reg

iste

r,w

hic

hco

mp

rise

d

0.10

%o

fp

eop

leo

ver

65y

ears

in

Lei

cest

ersh

ire.

No

dat

ag

iven

on

the

tota

ln

um

ber

of

peo

ple

aged

20–6

4y

ears

on

the

reg

iste

r.N

o

defi

nit

ion

of

LD

giv

en

Eac

hcl

ien

tw

asex

amin

edb

ya

psy

chia

tris

t

An

ov

eral

lra

teo

fp

sych

iatr

icd

iso

rder

for

eld

erly

(E),

61.9

%;

yo

un

g(Y

),

43.8

%.

Dep

ress

ion

(E6.

0%),

(Y4.

1%);

sch

izo

ph

ren

ia(E

3.0%

),(Y

2.7%

);

beh

avio

ura

lp

rob

lem

s(E

14.9

%),

(Y15

.1%

);m

ania

(E0.

7%),

(Y0%

);au

t-

ism

(E6.

0%),

(Y6.

8%);

Ret

ts(E

0%),

(Y1.

4%);

gen

eral

ized

anx

iety

(E9.

0%),

(Y5.

5%);

ago

rap

ho

bia

(E3.

7%),

(Y

1.4%

);o

ther

ph

ob

ias

(E3.

0)(Y

6.8%

);

dem

enti

a(E

21.6

%),

(Y2.

7%);

po

ssib

le

dem

enti

a(E

2.5%

),(Y

1.4%

);al

coh

ol

(E,

0%),

(Y,

1.4%

);O

CD

(E0%

),

(Y2.

7%)

Co

rbet

t(1

979)

402

adu

lts

wit

hM

R,

aged

15y

ears

and

old

er,

wh

ow

ere

on

the

Cam

b-

erw

ell

Reg

iste

ro

n31

Dec

1971

.

Th

esa

mp

leco

nsi

sted

of

0.28

%o

f

peo

ple

ov

er15

inC

amb

erw

ell.

No

clea

rd

efin

itio

no

fM

Rw

asg

iven

Fo

llo

win

gsc

reen

ing

asse

ssm

ent,

clie

nts

wer

ese

enfo

rp

sych

iatr

ic

exam

inat

ion

and

thei

rre

cord

s

exam

ined

An

ov

eral

lra

teo

fp

sych

iatr

icd

iso

rder

of

46.3

%.

Dep

ress

ion

,3.

0%;

sch

izo

-

ph

ren

ia,

6.2%

;b

ehav

iou

ral

pro

ble

ms,

25.4

%;

man

icd

epre

ssio

n,

2.2%

;ch

ild

-

ho

od

psy

cho

sis,

8.2%

Deb

etal

.(2

001)

Asa

mp

le90

adu

lts

aged

16–

64y

ears

wit

hm

ild

inte

llec

tual

dis

abil

itie

sw

asta

ken

fro

mth

e24

6

adu

lts

on

the

Val

eo

fG

lam

org

an

soci

alse

rvic

esca

sere

gis

ter,

all

liv

ing

inth

eco

mm

un

ity

.T

he

reg

iste

rco

mp

rise

d0.

32%

of

the

peo

ple

aged

16–6

4y

ears

.N

ocl

ear

defi

nit

ion

of

inte

llec

tual

dis

abil

itie

s

giv

en

Init

iall

ysc

reen

edo

nth

eM

ini

PA

S-A

DD

then

inte

rvie

wed

by

a

psy

chia

tris

tu

sin

gth

em

ain

PA

S-

AD

Dto

reac

han

ICD

-10

dia

gn

osi

s

An

ov

eral

lra

teo

f14

.4%

.D

epre

ssio

n,

2.2%

;sc

hiz

op

hre

nia

,4.

4%;

del

usi

on

al

dis

ord

er,

1.1%

;b

ipo

lar

dis

ord

er,

0%;

gen

eral

ized

anx

iety

,2.

2%;

ph

ob

icd

is-

ord

er,

4.4%



Tab

le2

Co

nti

nu

ed

Stu

dyD

escr

ipti

onof

sam

ple

How

diag

nos

isw

asob

tain

edR

ates

ofps

ychi

atri

cdi

sord

erre

port

ed

Dy

gg

ve

&K

od

ahl

(197

9)T

hes

ew

ere

the

942

peo

ple

,o

fal

lag

es,

reg

iste

red

inan

area

of

Dem

ark

on

1Ja

n

1973

asM

R,

0.36

%o

fth

ep

op

ula

tio

no

f

260

000.

Th

isin

clu

ded

all

pat

ien

tsw

ith

IQs

bel

ow

75,

ho

wev

er,

50%

of

them

had

IQ<

50

Th

ep

aper

did

no

tm

ake

this

clea

rS

ever

ep

rob

lem

beh

avio

urs

,8.

1%;

chil

dh

oo

dau

tism

abo

ut

2.2%

;

psy

cho

sis,

10%

Eat

on

&M

eno

lasc

ino

(198

2)79

8p

eop

le,

aged

6–76

yea

rs,

ina

com

mu

nit

yp

rog

ram

me

for

peo

ple

wit

h

MR

con

ver

gin

gfi

ve

cou

nti

esin

Neb

rask

a,

wh

ow

ere

des

crib

edas

fun

ctio

nin

gin

the

MR

ran

ge

by

anin

terd

isci

pli

nar

yte

am.

Th

eyco

mp

rise

d0.

17%

of

the

tota

l

po

pu

lati

on

of

482

000.

Of

thes

e,16

8

(21.

1%)

wer

ere

ferr

edto

psy

chia

try

bet

wee

nJu

ne

1976

and

Jun

e19

79

By

psy

chia

tric

asse

ssm

ent

foll

ow

ing

refe

rral

,b

ased

on

DS

M-I

II

An

ov

eral

lra

teo

f14

.2%

.D

epre

ssio

n,

0.0%

;sc

hiz

op

hre

nia

,3.

0%;

per

son

alit

y

dis

ord

er,

3.9%

;p

sych

on

euro

tic

anx

iety

,0.

1%;

adju

stm

ent

reac

tio

n,

3.0%

;o

rgan

icb

rain

syn

dro

me

4.3%

Far

mer

etal

.(1

993)

Th

e31

85p

eop

leo

nth

eN

WT

ham

es

Reg

ion

alH

ealt

hA

uth

ori

ty(R

HA

)re

gis

ter

for

wh

om

dia

gn

ost

icin

form

atio

nw

as

avai

lab

le.

NW

Th

ames

RH

Ah

ada

po

pu

lati

on

of

2.69

mil

lio

nab

ou

t5.

3%o

f

the

po

pu

lati

on

of

En

gla

nd

and

Wal

es,

the

reg

iste

rh

ad0.

23%

of

the

po

pu

lati

on

on

it.

Th

eysu

gg

est

the

tru

ep

rev

alen

cera

te

is0.

40%

.B

ein

go

nth

ere

gis

ter

occ

urr

ed

for:

‘th

ose

wh

ou

sed

on

eo

rm

ore

of

the

spec

iali

stse

rvic

esfo

rth

em

enta

lly

han

dic

app

ed,

or

wh

ow

ere

tho

ug

ht

lik

ely

tore

qu

ire

such

ase

rvic

e’

Th

ep

aper

did

no

tm

ake

this

clea

rA

no

ver

all

rate

of

17.8

%.

Dep

ress

ion

,

0.6%

;sc

hiz

op

hre

nia

,5.

3%;

per

son

alit

y

dis

ord

er,

1.3%

;au

tism

,2.

1%;

sen

ile

and

pre

-sen

ile

org

anic

psy

cho

sis,

0.2%

;al

coh

oli

cp

sych

osi

s,0%

;af

fect

ive

psy

cho

sis,

0.8%

;p

aran

oid

stat

es,

0%;

oth

ero

rgan

icp

sych

osi

s,1.

9%;

psy

cho

ses

wit

ho

rig

inin

chil

dh

oo

d,

5.0%

;n

euro

tic

dis

ord

ers,

1.3%

;se

xu

al

dev

iati

on

and

dis

ord

er,

0.1%

;sp

ecia

l

sig

ns

and

sym

pto

ms,

0.2%

;

no

n-p

sych

oti

caf

ter

bra

ind

amag

e,

0.2%

;d

istu

rban

ceo

fco

nd

uct

,0.

7%;

dis

turb

ance

of

emo

tio

n,

0.4%

Go

stas

on

(198

5)11

5p

eop

leag

ed20

–60

yea

rs,

wh

oco

uld

be

inte

rvie

wed

fro

mo

ut

of

132

peo

ple

iden

tifi

edas

hav

ing

MR

ina

Sw

edis

h

cou

nty

.T

hey

com

pri

sed

0.89

%o

fth

atag

e

gro

up

inth

eco

un

tyw

hic

hw

asfe

ltto

be

rep

rese

nta

tiv

eo

fS

wed

enas

aw

ho

le.

Th

eyd

efin

edM

Ras

hav

ing

anIQ

of

<74

on

the

Sw

edis

hte

sto

fin

tell

igen

ce(S

PIQ

)

Ap

sych

iatr

icin

terv

iew

wit

hth

e

clie

nt

and

/o

rin

form

ants

and

exam

inat

ion

of

case

no

tes

An

ov

eral

lra

teo

f33

.9%

had

atle

ast

a

mo

der

ate

and

defi

nit

em

enta

lil

lnes

s.

Sch

izo

ph

ren

ia,

2.6%

;p

erso

nal

ity

dis

ord

er,

3.5%

;st

utt

erin

g,

12.2

%;

ster

eoty

pic

mo

vem

ent,

10.4

%;

org

anic

men

tal

dis

ord

er,

26.1

%;

aty

pic

alp

ara-

no

idd

iso

rder

,0.

9%;

affe

ctiv

ed

iso

rder

s,

2.6%

,an

xie

tyd

iso

rder

,5.

2%(p

ho

bia

s,

0.9%

;O

CD

,0.

9%;

aty

pic

al3.

5%)

adju

stm

ent

dis

ord

er,

0.9%



Tab

le2

Co

nti

nu

ed

Stu

dyD

escr

ipti

onof

sam

ple

How

diag

nos

isw

asob

tain

edR

ates

ofps

ychi

atri

cdi

sord

erre

port

ed

Iver

son

&F

ox

(198

9)15

6p

eop

lew

ho

wer

ea

sam

ple

of

the

417

adu

lts

(ag

ed21

yea

rsan

do

ver

)w

ith

MR

kn

ow

nab

ou

tin

aU

Sco

un

tyw

ith

ap

op

-

ula

tio

no

f28

0,00

0.A

sab

ou

t70

%o

fth

e

US

po

pu

lati

on

iso

ver

20(b

ased

on

2000

US

cen

sus)

this

is0.

21%

of

the

po

pu

lati

on

of

the

cou

nty

.D

efin

itio

no

fle

arn

ing

dis

-

abil

ity

was

inli

ne

wit

hA

AM

Rd

efin

itio

ns

of

men

tal

reta

rdat

ion

Th

eP

sych

op

ath

olo

gy

Inst

rum

ent

for

Men

tall

yR

etar

ded

Ad

ult

s(P

IMR

A)

was

com

ple

ted

by

peo

ple

wh

oh

adk

no

wth

e

clie

nt

for

atle

ast

3m

on

ths

An

ov

eral

lra

teo

f35

.9%

wer

ecl

assi

fied

wit

hp

sych

op

ath

olo

gy

Jaco

bso

n(1

982)

Ab

ou

t32

500

peo

ple

(ch

ild

ren

and

adu

lts)

reg

iste

red

on

the

New

Yo

rkS

tate

reg

iste

r

of

peo

ple

rece

ivin

gse

rvic

esfo

rD

evel

op

-

men

tal

Dis

abil

itie

san

dfo

rw

ho

ma

cur-

ren

tcl

assi

fica

tio

no

fin

tell

ectu

al

fun

ctio

nin

gw

asav

aila

ble

.It

com

pri

sed

0.16

%o

fth

eto

tal

po

pu

lati

on

of

1897

645

7(b

ased

on

2000

cen

sus)

Th

ep

aper

did

no

tm

ake

this

clea

rA

no

ver

all

rate

of

11.6

%h

ada

psy

chia

tric

dis

abil

ity

.P

erso

nal

ity

dis

ord

ers,

2.1%

;

beh

avio

ura

lp

rob

lem

s,59

.7%

;p

sych

osi

s,

3.3%

;n

on

-psy

cho

tic

org

anic

bra

insy

n-

dro

mes

,1.

6%;

auti

sm,

1.5%

;o

ther

con

di-

tio

ns,

4.7%

Jaco

bso

n(1

990)

4247

9p

eop

le(c

hil

dre

nan

dad

ult

s)

reg

iste

red

on

the

New

Yo

rkS

tate

reg

iste

r

of

peo

ple

rece

ivin

gse

rvic

esfo

rD

evel

op

-

men

tal

Dis

abil

itie

s.It

com

pri

sed

0.22

%o

f

the

tota

lp

op

ula

tio

no

f18

976

000

(bas

ed

on

2000

US

cen

sus)

Info

rmat

ion

on

psy

chia

tric

dis

ord

erw

as

ob

tain

edb

ased

on

med

ical

or

psy

cho

log

i-

cal

do

cum

enta

tio

nin

the

ind

ivid

ual

sp

ro-

gra

mp

lan

An

ov

eral

lra

teo

f20

.0%

had

ap

sych

iatr

ic

dis

abil

ity

.P

erso

nal

lyd

iso

rder

s,2.

9%;

chil

dh

oo

db

ehav

iou

ral

dis

ord

er,

2.3%

;

psy

cho

sis,

4.3%

;n

on

-psy

cho

tic

org

anic

bra

insy

nd

rom

e,1.

6%;

neu

rosi

s,1.

4%

Lu

nd

(198

5)30

2p

eop

leag

ed20

yea

rsan

do

ver

,fr

om

aco

un

tyin

Den

mar

ko

nth

ere

gis

ter

of

MR

wh

ich

con

tain

ed22

500

nam

es(o

r

0.43

%o

fth

ep

op

ula

tio

nin

clu

din

gso

me

wit

hIQ

ov

er70

).T

he

sam

ple

cam

efr

om

asi

ng

leco

un

ty.

Defi

nit

ion

of

MR

was

bas

edo

nth

eW

HO

crit

eria

All

the

clie

nts

wer

eex

amin

edan

d

par

ents

or

staf

fin

terv

iew

edb

yth

e

auth

or,

ap

sych

iatr

ist

An

ov

eral

lra

teo

f28

.1%

had

ap

sych

iatr

ic

dis

ord

er.

Sch

izo

ph

ren

ia,

1.3%

;b

eha-

vio

ura

ld

iso

rder

s,10

.9%

;af

fect

ive

dis

or-

der

,1.

7%,

psy

cho

sis

of

un

cert

ain

typ

e,

5%;

neu

rosi

s,2%

;d

emen

tia,

3.6%

(22.

2%

for

tho

seag

ed65

+y

ears

),ea

rly

chil

dh

oo

d

auti

sm,

3.6%

;su

bst

ance

abu

sed

iso

rder

,

0.0%

Ro

jah

net

al.

(199

3)

(Cal

ifo

rnia

)

8941

9p

eop

leag

e0–

45y

ears

on

the

Cal

ifo

rnia

Dep

artm

ent

of

Dev

elo

pm

enta

l

Ser

vic

esre

gis

ter.

Th

isco

mp

rise

d0.

38%

of

the

po

pu

lati

on

of

0–45

-yea

ro

lds

in

Cal

ifo

rnia

Dia

gn

osi

sw

asm

ade

by

aq

ual

ified

pro

fess

ion

alm

akin

gu

seo

fT

he

Cli

ent

Dev

elo

pm

enta

lE

val

uat

ion

Rep

ort

(CD

ER

)

3.9%

had

aD

SM

-III

-Rd

iag

no

sis.

Sch

izo

ph

ren

ia,

1.3%

;p

erso

nal

ity

dis

ord

er

0.3%

;b

ehav

iou

ral

pro

ble

ms,

21.1

%;

AD

HD

,0.

6%;

con

du

ctD

,0.

4%;

per

vas

ive

dev

elo

pm

enta

lD

,0.

3%;

adju

stm

ent

D,

0.4%

;an

xie

tyD

,0.

3%;

OB

S,

0.1%

;af

fect

-

ive

dis

ord

er,

0.7%



with intellectual disabilities is higher than it is in chil-

dren without an intellectual disability.

There is also evidence that the rate of mental disorder

is higher for children with a greater degree of intellec-

tual disability, with Rutter et al. (1970), Stromme & Dis-

eth (2000) and Gillberg et al. (1986) finding higher rates

of psychiatric disorders among those with IQ < 50, or

< 60 (Birch et al. 1970). In addition, several studies

reported a relationship between either neurological con-

ditions (Birch et al. 1970; Rutter et al. 1970), a clear biolo-

gical aetiology to the intellectual disability

‘biopathological aetiology’ (Stromme & Diseth 2000) and

psychiatric disorders and higher rates of psychiatric dis-

orders.

Although there is some consistency in the overall rate

of psychiatric disorder reported in the studies, there is

considerable variation between the studies as to the

individual conditions that are reported and the preva-

lence of these conditions. It is therefore difficult to draw

any definitive conclusion as to what conditions are more

likely in children with intellectual disabilities. Nonethe-

less, there are some indications. On the whole, the con-

ditions reported on in Table 1 are similar to those

reported in the epidemiological studies of mental illness

in children who do not have an intellectual disability.

The exception to this are the pervasive developmental

disorders (PDD) which are not reported in non-intellec-

tual disability studies but are in four of the studies in

Table 1, with rates of 7.6% and 8% reported by Emerson

(2003) and Stromme & Diseth (2000) respectively.

The other more commonly reported conditions were

depression, anxiety disorders and behavioural disorders.

In Table 1, the rate of depression varied between 1.5%

reported by Emerson (2003) and 11% found by Linna

et al. (1999). Linna et al. (1999) also reported a 6.6% rate

in a control group of children without intellectual dis-

abilities; however, this was not statistically significantly

lower than the rate in the intellectual disability group.

The rates of depression found in Table 1 are also similar

to those reported in the studies of children without an

intellectual disability (1.5%, Costello et al. 1996; 1.8%,

Anderson et al. 1987; and 5.9%, Bird et al. 1988). The

studies therefore do not provide evidence that the pre-

valence of depression is higher among children with

intellectual disabilities than those without, a finding that

concurs with that of a recent review of epidemiological

studies of mood disorders in people with intellectual

disabilities by Rojahn & Esbensen (2005).

For children with an intellectual disability, anxiety

disorder varied from 21.9% (Dekker & Koot 2003) to

8.7% (Emerson 2003). These rates seem to be higher thanTab

le2

Co

nti

nu

ed

Stu

dyD

escr

ipti

onof

sam

ple

How

diag

nos

isw

asob

tain

edR

ates

ofps

ychi

atri

cdi

sord

erre

port

ed

Ro

jah

net

al.

(199

3)

(New

Yo

rk)

4568

3p

eop

leag

ed0–

45y

ears

on

the

New

Yo

rkS

tate

Offi

ceo

fM

enta

lR

etar

da-

tio

nan

dD

evel

op

men

tal

Dis

abil

itie

sre

gis

-

ter.

Th

isco

mp

rise

d0.

36%

of

the

po

pu

lati

on

of

0–45

-yea

ro

lds

inC

alif

orn

ia

Dia

gn

osi

sm

ade

by

qu

alifi

edm

edic

al

or

psy

chia

tric

staf

fm

akin

gu

seo

fT

he

Dev

elo

pm

enta

lD

isab

ilit

ies

Info

rmat

ion

Su

rvey

(DD

IS)

16.6

%co

nsi

der

edto

hav

ep

sych

iatr

ic

pro

ble

ms

on

ly5.

4%h

adD

SM

-III

-Rd

iag

-

no

sis.

Sch

izo

ph

ren

ia,

0.4%

;p

erso

nal

ity

dis

ord

ers,

0.2%

;b

ehav

iora

lp

rob

lem

s,

40.1

%;

AD

HD

,0.

3%;

con

du

ctD

,1.

2%;

per

vas

ive

dev

elo

pm

enta

lD

,1.

7%;

adju

st-

men

tD

,0.

3%;

anx

iety

D,

0.5%

;O

BS

,

0.8%

;af

fect

ive

dis

ord

er,

0.0%

MR

,m

enta

lre

tard

atio

n;

OC

D,

ob

sess

ive–

com

pu

lsiv

ed

iso

rder

;A

DH

D,

atte

nti

on

defi

cit

hy

per

acti

vit

yd

iso

rder

;O

BS

,o

rgan

icb

rain

syn

dro

me;

WH

O,

Wo

rld

Hea

lth

Org

anis

atio

n;

PA

S-A

DD

,P

sych

iatr

icA

sses

smen

tS

ched

ule

for

Ad

ult

sw

ith

Dev

elo

pm

enta

lD

isab

ilit

ies,



those reported in the studies of children without an

intellectual disability – 5.7% (Costello et al. 1996) and

6.4% for separation anxiety and simple phobias com-

bined (Anderson et al. 1987).

All the studies presented in Table 1 that give the pre-

valence of individual conditions report on some form of

problematic behaviours such as ‘disruptive behaviour’

(25.1%, Dekker & Koot 2003), ‘conduct disorder’ (25.0%,

Emerson 2003; 7.9%, Gillberg et al. 1986; 3%, Stromme &

Diseth 2000) or ‘behavioural disturbance’ (22.5%, Linna

et al. 1999). These seem to be higher rates than those

reported in the non-intellectual disability studies, with

Costello et al. (1996) reporting 6.6% for any behaviour

disorder and Anderson et al. (1987) reporting 3.4% for

conduct disorder and 5.7% oppositional disorder. Hence

there is some indication that the increase in overall rate

of psychiatric disorder in children with an intellectual

disability is due to PDD and behavioural disorders, and

some increased rate of anxiety.

Studies of adults

These studies are presented in Table 2. Only two studies

(Borthwick-Duffy & Eyman 1990 and Iverson & Fox

1989) used a dual criterion for having an intellectual dis-

ability; however, their samples corresponded to only

0.23% and 0.21% of the population as a whole respect-

ively. This is less than one would expect if the sample

was a true random sample of people in the population

who would meet a dual criterion. Gostason (1985) uses

a criterion based on IQ < 74. All other studies used

pragmatic criteria based on being on a register of people

with intellectual disabilities or receiving services, so are

administrative samples. The proportion of the popula-

tion represented in the samples ranged from 0.10%

(Cooper 1997) for older adults, to 0.89% by Gostason

(1985) for people aged 20–60 years with IQs <74.

Although Gostason (1985) used a sample corresponding

to nearly 1% of the population – the level suggested as

the minimum prevalence rate for people with intellec-

tual disabilities – the sample was of people with intellec-

tual disabilities with IQs <74, which should be 4.2% of

the population. However, the Gostason (1985) study is

of particular significance as it used a control group of

people without intellectual disabilities, matched to gen-

der and location.

Only Corbett (1979) and Deb et al. (2001) are explicit

about what type of prevalence they reported on. This

represents a considerable limitation of these studies as

similar studies of the prevalence of mental illness in the

general population (e.g. Bland et al. 1988a,b) have found

a large difference between point and lifetime preval-

ence.

Although, in most of the studies, the client was exam-

ined as part of the study, this was not the case in all

studies. For some (Dyggve & Kodahl 1979; Eaton &

Menolascino 1982; Jacobson 1982, 1990; Farmer et al.

1993; Rojahn et al. 1993) data were taken from the cli-

ents’ records or from a register. This introduces several

possible sources of error. First, there is a higher chance

of error in the transferring of data to and from the regis-

ters. Second, it is unlikely that everyone in the sample

would have been systematically examined by a qualified

professional, but rather only those who were thought to

have a mental illness by their carers would be referred

for an assessment. There is therefore the possibility of

diagnostic overshadowing (Reiss 1994, 2000), the process

by which the symptoms of psychiatric conditions are

attributed to a direct effect of the intellectual disability

and so the diagnosis is missed.

Thirteen of the 14 studies in Table 2 give an overall

rate of psychiatric disorder. However, in many of the

studies, the rate does not differ significantly from that

reported by those studies reporting the overall rate of

mental illness in the population as a whole. For exam-

ple, Eaton & Menolascino (1982), Jacobson (1982, 1990)

Borthwick-Duffy & Eyman (1990), Farmer et al. (1993),

Deb et al. (2001), and Rojahn et al. (1993) all report over-

all rates similar to, or less than, the 17.1% 6-month pre-

valence reported by Bland et al. (1988b). Most of the

studies that report higher overall rates used administra-

tive samples. For example, the proportion of the popula-

tion as a whole in the samples were: Cooper (1997) only

0.1% of people >65 years old; Corbett (1979) 0.28% of

people >15 years, and Lund (1985) only 0.43% of people

aged ‡20 years. The Gostason (1985) study is notable in

that it made use of a control group. It was found that

although there was no significant difference between

people with mild intellectual disability and the control

group, in the rate of psychiatric disorders, people with

severe intellectual disability were significantly more

likely to have at least one DSM-III diagnosis. The study

therefore does provide evidence that the rate of psychi-

atric disorder is greater in people with severe intellec-

tual disability.

As with the studies of children, there is a lack of con-

sistency as to the conditions reported on in the studies.

Some do not give any data for individual conditions

(Borthwick-Duffy & Eyman 1990, and Iverson & Fox

1989) but simply report an overall rate of psychiatric

disorder. Some use broad categories of psychiatric dis-

orders such as psychosis, rather than more specific



conditions such as schizophrenia (e.g. Dyggve & Kodahl

1979; Jacobson 1982, 1990). Others (Gostason 1985;

Farmer et al. 1993; Cooper 1997) report a large number

of different conditions. Some specific conditions are

reported on in a number of studies: schizophrenia, in

nine of the studies; depression or affective disorders,

in seven of the studies; and personality disorders, in

seven of the studies. In addition, seven of the studies

report on the prevalence of ‘behaviour problems’. Only

three studies report PDDs.

Behavioural problems

Although ‘behavioural problems’ are not a psychiatric

disorder as such, they have been included in a number

of studies in Table 2, and appear to make a substantial

contribution to the relatively high overall rates of psy-

chiatric disorder reported in several of the studies. Coo-

per (1997) reported overall rates of 61.9% for her elderly

group and 43.8% for the young group with rates for

behavioural problems for these groups being 14.9% and

15.1% respectively. Corbett (1979) reported an overall

rate of psychiatric disorders of 46.3% and a rate of beha-

vioural problems of 25.4%. Lund (1985) reports an over-

all rate of mental illness of 28.1% and that of

behavioural problems of 10.9%. It is therefore likely that

an additional reason for the higher rates of psychiatric

disorders in most of the studies that found high rates is

the inclusion of behavioural problems as a condition.

There is good evidence from other studies that the rate

of behavioural problems is higher in people with intel-

lectual disabilities than in the population as a whole.

For example, Koller et al. (1982, 1983) followed up the

children in the original Birch et al. (1970) study when

they were young adults and found that the prevalence

of ‘behavioural disturbance’ was significantly greater in

the adults with intellectual disabilities than in non-intel-

lectual disability controls matched to age, gender and

social class. However, it is not clear if behavioural prob-

lems as such should be considered to be a psychiatric

disorder.

Schizophrenia

DSM-IV-R gives the prevalence of schizophrenia in the

population as a whole as between 0.5% and 1.5%. The

Edmonton study found a lifetime prevalence of 0.6%

(Bland et al. 1988a), and a 6-month period prevalence of

0.3% (Bland et al. 1988b), and Myers et al. (1984) report a

6-month prevalence rate of schizophrenia or schizo-

pheniform disorders of 0.97%. The prevalence in people

with intellectual disabilities is usually considered to be

higher (Tyrer & Dunstan 1997; Doody et al. 1998). In a

review of schizophrenia in people with intellectual dis-

abilities, Turner (1989) suggests that the rate was about

3%. The studies in Table 2 would appear, at face value,

to support this with reported rates for schizophrenia

ranging from 5.1% (Farmer et al. 1993) to 1% (Gillberg

et al. 1986). However, these were all administrative sam-

ples. It is likely that having schizophrenia would bring

a client to the attention of services, resulting in them not

only being identified as having schizophrenia but also

as having a learning disability. It is therefore unsurpris-

ing that an administrative sample of people with intel-

lectual disabilities contains a higher rate of people who

also have schizophrenia. Gostason (1985) is of interest

here as, although an administrative sample of people

with intellectual disabilities was used, the study also

employed a control group of people in the general pop-

ulation and found no statistically significant difference

between the rate of schizophrenia in people with mild

intellectual disability and the controls. Therefore, on the

basis of the studies in Table 2, there is no compelling

evidence that the prevalence of schizophrenia is any

higher in people with intellectual disabilities than it is

in the rest of the population. However, there is other

evidence that suggests that prevalence may be higher.

David et al. (1997) looked back at the results of intelli-

gence tests that had been carried out on 195 men suffer-

ing from schizophrenia, when they had been

conscripted into the Swedish army 13 years previously.

It was found that the rates of schizophrenia was 1.3%

for those with IQ < 74 compared with 0.6% for IQ 74–

95, and 0.3% for IQ 96–126, providing good evidence

that the rate of schizophrenia is higher in people with

intellectual disabilities.

Depression

In the population as a whole, DSM-IV-R suggests that

lifetime risk of a major depressive disorder is 10–25%

for women and 5–15% for men. The Edmonton study

reports a lifetime prevalence of affective disorders

(which includes depression as well as mania; Gelder

et al. 1989) of 10.2% (Bland et al. 1988a) and a 6-month

period prevalence of 5.7% (Bland et al. 1988b). In Korea,

Lee (1998) reports a lifetime prevalence of 5.37%. The

only study in Table 2 that found a higher rate than

Bland et al. (1988a,b) was Cooper (1997), who reports

rates of 6% in an administrative sample of elderly peo-

ple with intellectual disabilities. Indeed, several studies

found rates of <1% or 0. Therefore, on the basis of the



studies in Table 2, the rate of depression may well be

lower in people with intellectual disabilities than it is in

the mainstream population. This is in line with the con-

clusion of reviews of depression in people with intellec-

tual disabilities (Davis et al. 1997; Rojahn & Esbensen

2005). However, another study does suggest that the

rate is higher. Richards et al. (2001) found that the 143

adults, from a birth cohort of 4017, who had been

assessed as having IQs between 70 and 50 at age

15 years, had a fourfold greater rate of affective disorder

that the cohort as a whole.

Personality disorders

Seven of the studies in Table 2 report personality disor-

ders as a single diagnostic entirety, with prevalence

rates ranging between 3.9% (Eaton & Menolascino

1982) and 0.2% (Rojahn et al. 1993). All these studies

use administrative samples, which again may be biased

as personality disorder is characterized by exocentric

behaviour that would tend to draw attention to the

person and result in their intellectual disability being

identified. The authors are not aware of any studies

that report on the rate of personality disorder as a sin-

gle entity in the population as a whole, however, a

number of studies have reported on the rate of antiso-

cial personality disorder. DSM-IV-R suggests that in a

community sample it is about 3% in males and 1% in

females. Bland et al. (1988b)) report a 6-month preval-

ence of 1.8%, Myers et al. (1984) found an average 6-

month prevalence to be 0.8% in three US cities and

Lee (1998) found a lifetime prevalence of 1.63% in

Korea. These figures do not seem greatly different from

the rates of personality disorder as a whole reported in

Table 2. Therefore, on the current evidence, it is not

clear that the rate of personality disorders is any

higher in people with intellectual disabilities than it is

in the general population.

Discussion

The literature on the prevalence of mental illness in peo-

ple with intellectual disabilities has a number of short-

comings.

All the studies on adults and many of those on chil-

dren use administrative samples, which cannot be con-

sidered random. There is a distinct possibility that it

was the presence of the psychiatric disorder that led to

some of the individuals in the sample being identified

as having an intellectual disability. The samples may

therefore contain a higher proportion of people with a

psychiatric disorder than is the case for people with

intellectual disabilities as a whole.

Only four studies were explicit as to what type of pre-

valence they were reporting on. This represents a con-

siderable limitation of these studies as similar studies of

the prevalence of psychiatric disorders in the general

population (e.g. Bland et al. 1988a,b) have found a large

difference between point and lifetime prevalence.

There is considerable variation between the studies as

to how the diagnosis was obtained. All the studies on

children in Table 1 and several of the studies in Table 2

examined the client specifically for the study; however,

a number of studies (Eaton & Menolascino 1982; Jacob-

son 1982, 1990; Borthwick-Duffy & Eyman 1990; Farmer

et al. 1993, and Rojahn et al. 1993) took data from a

register or from the clients’ records. This latter method

could be subject to error when information is transferred

either onto or from a record. Moreover, it is unlikely

that everyone in the sample would have been systemat-

ically examined by a qualified professional, but rather

only those who were thought to have a mental illness

by their carers would be referred for an assessment.

There is therefore the possibility of diagnostic overshad-

owing (Reiss 1994, 2000), the process by which the

symptoms of psychiatric conditions are attributed to a

direct effect of the intellectual disability and so the diag-

nosis is missed. In this respect it is notable that the stud-

ies that report lower overall rate of psychiatric disorders

obtained the information from pre-existing records.

Only four studies (Gostason 1985; Linna et al. 1999;

Dekker et al. 2002 and Rutter et al. 1970) make use of a

control group of non-intellectual disability people. Of

these, only Gostason (1985) used a matched control

group, although even then did not match for all varia-

bles that could influence the prevalence of psychiatric

disorders such as social class. In addition to this, all

four studies assessed the clients with non-intellectual

disabilities in the same way as those with intellectual

disabilities. This seems to assume that the clients with

and without intellectual disabilities express mental ill-

ness in the same way, which may not be the case. This

highlights a further problem with much of the research

reported here, that of a lack of demonstrable reliability

in diagnoses that are made. Psychiatric diagnosis is lar-

gely based on clinical judgement as to whether the

behaviours shown by the patient are indicative of a

specific condition. There is therefore scope for error,

which, if systematic, could lead to an underestimate or

overestimate of the proportion of people in a popula-

tion who suffer from the condition. It seems likely that

these errors will be increased if the manner in which a



condition is presented differs between groups of

patients.

There is a lack of consistency between the studies as

to what constitutes a psychiatric disorder, with different

studies including different conditions. Therefore, the

studies reporting higher overall rates may be using a

wider definition of psychiatric disorders rather than

examining a population that has a greater prevalence of

psychiatric disorders. Most notable is how the studies

dealt with ‘behavioural problems’, a condition that is

not a psychiatric disorder in itself, not being listed as

such in DSM-IV-R. Behavioural problems make up a

considerable proportion of the overall rate of mental ill-

ness in both the Cooper (1997) and Lund (1985) studies

and so may be the reason for the apparent higher pre-

valence of mental illness.

In spite of these methodological problems, it is poss-

ible to draw some tentative conclusions with regard to

the prevalence of psychiatric disorders among people

with intellectual disabilities.

First, there is evidence that the overall prevalence of

psychiatric disorders is greater in children with intellec-

tual disabilities than for their peers with no intellectual

disabilities. Second, there is evidence from Birch et al.

(1970), Rutter et al. (1970), Gillberg et al. (1986), Gostason

(1985) and Stromme & Diseth (2000) that the rate of

mental illness is higher in both adults and children with

severe intellectual disability than either people with

mild intellectual disabilities or those without intellectual

disabilities. This finding is compatible with the hypoth-

esis that brain pathology is a predisposing factor for

psychiatric disorders. Third, there seems to be a higher

rate of behavioural problems among both adults and

children with intellectual disabilities than those without

intellectual disabilities. This is evident from the rela-

tively high rates reported in the studies in Tables 1 and

2, and from the controlled study by Koller et al. (1982).

There is, however, as yet no convincing evidence that

the overall rate of psychiatric disorders for adults with

mild intellectual disabilities (IQ 50–70) is any higher

than that for the population as a whole. It is therefore

unclear as to whether having a low IQ as such or the

effects of a low intellectual ability, such as a poor under-

standing of one’s own emotions or poor self-esteem,

predisposes people to develop psychiatric disorder. It is

also not possible to make definitive statements that the

rate of mental illness as a whole is higher in people with

intellectual disabilities.

The current lack of evidence linking mild intellectual

disabilities to a higher prevalence of psychiatric dis-

orders in adults should not be taken as conclusive proof

that the rate of psychiatric disorders is not higher in this

group. It is possible that the rate of psychiatric disorders

is higher but that this is not apparent from these studies.

This could be because either the studies have been meth-

odologically inadequate or because the symptoms of var-

ious psychiatric disorders are different in people with

intellectual disabilities than in the mainstream popula-

tions. The obvious possibility is that behavioural prob-

lems are more often symptomatic of mental illness in

this group. In the studies of children in Table 1, beha-

vioural problems were part of diagnosed conditions such

as conduct disorder, oppositional defiant disorder and

ADHD. However, these are diagnoses that are not nor-

mally used with adults. In the studies of adults listed in

Table 2, behavioural disorders seem to be classified sim-

ply as ‘behavioural disordered’ which is not a diagnostic

classification. It is possible that adults with intellectual

disabilities continue to have the same disturbed beha-

viour as they did as children. However, in adults, these

behaviours are not seen as symptomatic of specific psy-

chiatric conditions. With regard to this, it is notable that

when Koller et al. (1982, 1983) followed up the children

originally identified by Birch et al. (1970) rather than talk

about these adults as being diagnosed as with ‘psychi-

atric abnormality’, they referred to them as having ‘beha-

vioural disturbance’. However, it would be a mistake to

assume that all problem behaviours seen in people with

intellectual disabilities is symptomatic of psychiatric dis-

orders. There is good evidence in the literature on

applied behaviour analysis (e.g. Iwata et al. 1994), that

behavioural problems are explainable in terms of the

contingencies of reinforcement operating in their envi-

ronment. In addition, when the relationship between

problem behaviours and mental illness in people with

intellectual disabilities has been examined, the results

have been somewhat contradictory. Moss et al. (2000)

compared clients who showed challenging behaviour

with a control group on the PAS-ADD checklist and

found that clients who showed more severe challenging

behaviours had significantly more symptoms on the

PAS-ADD. In contrast, Rojahn et al. (1993) failed to find

compelling correlations between DSM-III-R diagnosis

and the presence of challenging behaviour in clients on

both the New York State and California registers of peo-

ple with mental retardation. It is not clear why these dif-

ferent results occurred. It is therefore as yet not clear to

what extent problem behaviour is symptomatic of psy-

chiatric disorders in people with intellectual disabilities.

There is clearly a need for further research. First,

understanding better how psychiatric disorders are

expressed in people with intellectual disabilities would



give insight into both the nature of psychiatric disorder

and the nature of intellectual disabilities. Until this work

is done, it will not be possible to make a true compar-

ison between the overall prevalence of psychiatric disor-

ders in the population with intellectual disabilities and

the mainstream population, as different diagnostic cri-

teria may need to be used with the different popula-

tions. However, investigating if particular behaviours

are symptoms of a psychiatric disorder may prove diffi-

cult. If a patient does not show the usual symptoms of a

psychiatric disorder, in the absence of biological con-

firmatory tests it may not be possible to say that the

patient has a psychiatric disorder, or what that psychi-

atric disorder is.

Second, there is a need for methodologically sound

studies regarding the prevalence of psychiatric disorders

in people with intellectual disabilities as a whole. This

may also be difficult, not only because it is unclear how

psychiatric disorders are expressed in people with intel-

lectual disabilities, but also because it would involve

identifying all the people with intellectual disabilities in

a total population, and then comparing the rate of psy-

chiatric disorders in this group with the rest of the pop-

ulation. One could either examine an entire population

of adults to identify those who had an intellectual dis-

ability in a similar way to how Rutter et al. (1970) and

Birch et al. (1970) did for children, or more realistically,

people who have already been identified in a cohort

study, such as that reported on by Emerson (2003),

could be followed up as adults.

Correspondence

Any correspondence should be directed to Simon Whi-

taker, The Learning Disability Research Unit, Room

HW02/8, The University of Huddersfield, Queensgate

Huddersfield, HD1 3DH, UK (e-mail: s.whitaker@hud.

ac.uk).

References

American Psychiatric Association (2000) Diagnostic and Statisti-

cal Manual of Mental Disorders, 4th edn, Text Revision. Ameri-

can Psychiatric Association, Washington DC.

Anderson J. C., Williams S., McGee R. & Silva P. A. (1987)

DSM-III disorders in preadolescent children: prevalence in a

large sample from the general population. Archives of General

Psychiatry 44, 69–76.

Ballinger C. B. (1997) Affective disorders. In: Psychiatry in Learn-

ing Disability (ed. S. G. Read), pp. 216–236. W. B. Saunders

Company Ltd, London.

Ballinger B. R. & Reid A. H. (1977) Psychiatric disorder in an

adult training centre and a hospital for the mentally handi-

capped. Psychological Medicine 7, 525–528.

Benson B. A. (1985) Behavior disorders and mental retardation:

Assessment with age, sex, and level of functioning in an out-

patient clinic sample. Applied Research in Mental Retardation 6,

79–85.

Berry P. & Gaedt C. (1995) Psychiatry of mental retardation.

Current Opinions in Psychiatry 8, 293–300.

Birch H. G., Richardson S. A., Baird D., Horobin G. & Illsley R.

(1970) A Clinical and Epidemiological Study: Mental Subnormal-

ity in the Community. The Williams and Wilkins Co, Balti-

more, MD.

Bird H. R., Canino G., Rubio-Stipec M., Gould M. S., Ribera

J., Sesman M., Woodbury M., Huertas-Goldman S., Pagan

A., Sanchez-Lacey A. & Moscoso M., (1988) Estimates of

the prevalence of childhood maladjustment in a community

survey in Puerto Rico. Archives of General Psychiatry 45,

1120–1126.

Bland R. C., Orn H. & Newman S. C. (1988a) Lifetime preva-

lence of psychiatric disorders in Edmonton. Acta Psychiatrica

Scandinavia 77 (Suppl. 338), 24–32.

Bland R. C., Newman S. C. & Orn H. (1988b) Period prevalence

of psychiatric disorders in Edmonton. Acta Psychiatrica Scan-

dinavia 77 (Suppl. 338), 33–42.

Bongiorno F. P. (1996) Dual diagnosis: developmental disability

complicated by mental illness. Southern Medical Journal 89,

1142–1146.

Borthwick-Duffy S. A. (1994) Epidemiology and prevalence of

psychopathology in people with mental retardation. Journal of

Consulting and Clinical Psychology 62, 17–27.

Borthwick-Duffy S. A. & Eyman R. K. (1990) Who are the

dually diagnosed? American Journal on Mental Retardation 94,

586–595.

Campbell M. & Malone R. P. (1991) Mental retardation and

psychiatric disorders. Hospital and Community Psychiatry 42,

374–379.

Chaplin R. (2004) General psychiatry services for adults with

intellectual disabilities and mental illness. Journal of Intellec-

tual Disability Research 48, 1–10.

Cooper S.-A. (1997) Psychiatry of elderly compared to younger

adults with intellectual disabilities. Journal of Applied Research

in Intellectual Disabilities 10, 303–311.

Corbett J. A. (1979) Psychiatric morbidity and mental retarda-

tion. In: Psychiatric Illness and Mental Handicap (eds F. E.

James & R. P. Snaith), pp. 11–25. Gaskill Press, London.

Costello E. J., Angold A., Burns B. J., Stangl D. K., Tweed D. L.,

Erkanli A. & Worthman C. M. (1996) The Great Smoky

Mountains study of youth: goals, design, methods and the

prevalence of DSM-III-R disorders. Archives of General Psychia-

try 53, 1129–1136.

David A. S., Malmberg A., Brandt L., Allebeck P. & Lewis G.

(1997) IQ risk for schizophrenia: a population-based cohort

study. Psychological Medicine 27, 1311–1323.



Davis J. P., Judd F. K. & Herrman H. (1997) Depression

in adults with intellectual disability. Part 1: a review.

Australian and New Zealand Journal of Psychiatry 31, 232–

242.

Day K. (1985) Psychiatric disorder in the middle-aged and

elderly mentally handicapped. British Journal of Psychiatry

147, 660–667.

Deb S., Thomas M. & Bright C. (2001) Mental disorder in adults

with intellectual disability. 1: Prevalence of functional psy-

chiatric illness among a community based sample population

aged between 16 and 64 years. Journal of Intellectual Disability

Research 45, 495–505.

Dekker M. C. & Koot H. M. (2003) DSM-IV disorders in chil-

dren with borderline to moderate intellectual disability. I:

Prevalence and impact. Journal of the Academy of Child and

Adolescent Psychiatry 42, 915–922.

Dekker M. C., Koot H. M., van der Ende J. & Verhulst F. C.

(2002) Emotional and behavioral problems in children and

adolescents with and without intellectual disability. Journal of

Child Psychology and Psychiatry 43, 1087–1098.

Demb H. B., Brier N., Huron R. & Tomor E. (1994) The Adoles-

cent Behavior Checklist: normative data and sensitivity and

specificity of a screening tool for diagnosable psychiatric dis-

orders in adolescents with mental retardation and other

developmental disabilities. Research in Developmental Disabil-

ities 15, 151–165.

Doody G. A., Johnstone E. C., Sanderson T. L., Cunningham

Ownens D. G. & Muir W. J. (1998) ‘Pfropfschizophrenie’ revis-

ited: schizophrenia in people with mild learning disability.

British Journal of Psychiatry 173, 145–153.

Dyggve H. & Kodahl T. (1979) Disease patterns in 942 mentally

retarded persons in a Danish county. Acta Psychiatrica Scandi-

navia 59, 381–394.

Eaton L. F. & Menolascino F. J. (1982) Psychiatric disorders

in the mentally retarded: types, problems, and challenges.

American Journal of Psychiatry 139, 1297–1303.

Einfeld S. L. & Tonge B. J. (1996a) Population prevalence of

psychopathology in children and adolescents with intellectual

disabilities: 1. Relationship and methods. Journal of Intellectual

Disability Research 40, 91–98.

Einfeld S. L. & Tonge B. J. (1996b) Population prevalence of

psychopathology in children and adolescents with intellectual

disabilities: II. Epidemiological findings. Journal of Intellectual

Disability Research 40, 99–109.

Emerson E. (2003) Prevalence of psychiatric disorders in chil-

dren and adolescents with and without intellectual disability.

Journal of Intellectual Disability Research 47, 51–58.

Farmer R., Rohde J. & Sacks B. (1993) Changing Services for

People with Learning Disabilities. Chapman and Hall, London.

Gelder M., Gath D. & Mayou R. (1989) Oxford Textbook of

Psychiatry, 2nd edn. Oxford University Press, Oxford.

Gillberg C., Presson E., Grufman M. & Themner U. (1986) Psy-

chiatric disorders in mildly and severely mentally retarded

urban children and adolescents: epidemiological aspects. Brit-

ish Journal of Psychiatry 149, 68–74.

Glue P. (1989) Rapid cycling affective disorder in the mentally

retarded. Biological Psychiatry 26, 250–256.

Gostason R. (1985) Psychiatric illness among the mentally retar-

ded: a Swedish population study. Acta Psychiatrica Scandina-

via 71 (Suppl. 381), 1–117.

Heaton-Ward A. (1977) Psychosis in mental handicap: The

Tenth Black Marsh Lecture delivered before the Royal

College of Psychiatrists, February 2, 1976. British Journal of


Iverson J. C. & Fox R. A. (1989) Prevalence of psychopathology

among mentally retarded adults. Research in Developmental

Disabilities 10, 77–83.

Iwata B. A., Pace G. M., Dorsey M. F., Zarcone J. R., Vollmer T.

R., Smith R. G., Rodgers T. A., Lerman D. C., Shore B. A.,

Mazaleski J. L., Goh H. L., Cowdery G. E., Kalsher M. J.,

McCosh K. C. & Willis K. D. (1994) The function of self-injur-

ious behavior: an experimental-epidemiological analysis. Jour-

nal of Applied Behavior Analysis 27, 215–240.

Jacobson J. W. (1982) Problem behavior and psychiatric impair-

ment within a developmentally disabled population 1: Behav-

ior frequency. Applied Research in Mental Retardation 3, 112–139.

Jacobson J. W. (1990) Do some mental disorders occur less fre-

quently among persons with mental retardation? American

Journal on Mental Retardation, 94, 596–602.

Jopp D. A. & Keys C. B. (2001) Diagnostic overshadowing

reviewed and reconsidered. American Journal on Mental Retar-

dation 106, 416–433.

Koller H., Richardson S. A., Katz M. & McLaren J. (1982) Beha-

vior disturbance in childhood and early adult years in popu-

lations who were and were not mentally retarded. Journal of

Preventive Psychiatry 1, 453–468.

Koller H., Richardson S. A., Katz M. & McLaren J. (1983) Beha-

vior disturbance since childhood among a 5-year birth cohort

of all mentally retarded young adults in a city. American Jour-

nal of Mental Deficiency 87, 386–395.

Kushlick A. (1975) Epidemiology and evaluation of services for

the mentally handicapped. In: The Mentally Retarded and Society:

A Social Science Perspective (eds M. J. Begab & S. A. Richardson),

pp. 325–343. University Park Press, Baltimore, MD.

Lee C. K. (1998) A nationwide epidemiological study of mental

disorder in Korea. Psychiatry and Clinical Neuroscience

52(Suppl.), S268–S274.

Linna S. -L., Moilanen I., Ebeling H., Piha J., Kumpulainen K.,

Tamminen T. & Almqvist F. (1999) Psychiatric symptoms

in children with intellectual disability. European Child and

Adolescent Psychiatry 8, 77–82.

Lund J. (1985) The prevalence of psychiatric morbidity in men-

tally retarded adults. Acta Psychiatrica Scandinavia 72, 563–

570.

Matson J. L., Barrett R. P. & Helsel W. J. (1988) Depression in

mentally retarded children. Research in Developmental Disabil-

ities 9, 39–46.

Menolascino F. J. & Fleisher M. H. (1991) Developmental con-

cepts in mental retardation and mental illness. Comprehensive

Mental Health Care 1, 45–56.



Molteno G., Molteno C. D., Finchilescu G. & Dawes A. R. L.

(2001) Behavioural and emotional problems in children with

intellectual disabilities attending special school in Cape Town.

South Africa Journal of Intellectual Disability Research 45, 515–520.

Moss S. (2001) Psychiatric disorders in adults with mental retar-

dation. In: International Review of Research in Mental Retardation

(ed. L. Glidden), pp. 211–243. Academic Press, New York.

Moss S. & Patel P. (1997) Dementia in older people with intel-

lectual disability: symptoms of physical and mental illness,

and levels of adaptive behaviour. Journal of Intellectual Disabil-

ity Research 41, 60–69.

Moss S. C., Emerson E., Kierman C., Turner S., Hatton C. &

Alborz A. (2000) Psychiatric symptoms in adults with learn-

ing disability (mental retardation) and challenging behaviour.

British Journal of Psychiatry 177, 452–456.

Myers B. A. (1987a) Conduct disorder of adolescents with

developmental disabilities. Mental Retardation 25, 335–340.

Myers B. A. (1987b) Psychiatric problems in adolescents with

developmental disabilities. Journal of the American Academy of

Child Psychiatry 26, 74–79.

Myers J. K., Weissman M. M., Tischler G. L., Holzer C. E., Leaf

P. J., Orvaschel H., Anthony J. C., Boyd J. H., Burke J. D.,

Kramer M. & Stoltzman R. (1984) Six-month prevalence of

psychiatric disorders in three communities. Archives of General


Patel P., Goldberg D. & Moss S. (1993) Psychiatric morbidity in

older people with moderate and severe learning disability II:

The prevalence study. British Journal of Psychiatry 163, 481–491.

Philips I. & Williams N. (1975) Psychopathology and mental

retardation: a study of 100 mentally retarded children: 1. Psy-

chopathology. American Journal of Psychiatry 132, 1265–1271.

Reid A. H. (1980) Psychiatric disorders in mentally handi-

capped children: A clinical and follow-up study. Journal of

Mental Deficiency Research 24, 287–298.

Reid A. & Ballinger B. R. (1987) Personality disorders in mental

handicap. Psychological Medicine 17, 983–987.

Reiss S. (1990) Prevalence of dual diagnosis in community-

based day programs in the Chicago metropolitan area. Ameri-

can Journal on Mental Retardation 94, 578–585.

Reiss S. (1994) Psychopathology in mental retardation. In: Men-

tal Health in Mental Retardation: Recent Advances in Practice

(ed. N. Bouras), pp. 67–78. Cambridge University Press, Cam-

bridge.

Reiss S. (2000) A mindful approach to mental retardation. Jour-

nal of Social Issues 56, 65–80.

Richards M., Maughan B., Hardy R., Hall I., Strydon A. &

Wadsworth M. (2001) Long-term affective disorder in people

with mild learning disability. British Journal of Psychiatry 179,

523–527.

Rojahn J. & Esbensen A. J. (2005) Epidemiology of mood disor-

ders in people with mental retardation. In: Mood Disorders in

Individuals with Mental Retardation (ed. P. Sturmey), pp. 47–66.

NADD Press, New York.

Rojahn J., Borthwick-Duffy S. A. & Jacobson J. W. (1993) The

association between psychiatric diagnosis and severe beha-

vior problems in mental retardation. Annals of Clinical Psy-

chiatry 5, 163–170.

Rutter M., Tizard J. & Whitmore K. (1970) Education, Health and

Behaviour. Longman, London.

Stromme P. & Diseth T. H. (2000) Prevalence of psychiatric

diagnoses in children with mental retardation: data from a

population-based study. Developmental Medicine and Child

Neurology 42, 266–270.

Turner T. H. (1989) Schizophrenia and mental handicap: an his-

torical review, with implications for further research. Psycho-

logical Medicine 19, 301–314.

Tyrer S. P. & Dunstan J. A. (1997) Schizophrenia. In: Psychiatry

in Learning Disability (ed. S. G. Read), pp. 185–215. W.B. Saun-

ders Company Ltd, London.

Vitiello B. & Behar D. (1992) Mental retardation and psychiatric

illness. Hospital and Community Psychiatry 43, 494–499.

Vitiello B., Spreat S. & Behar D. (1989) Obsessional-compulsive

disorder in mentally retarded patients. The Journal of Nervous

and Mental Disease 177, 232–236.

Whitaker S. (2004) Hidden learning disability. British Journal of

Learning Disability 32, 139–143.

Wright E. C. (1982) The presentation of mental illness in men-

tally retarded adults. British Journal of Psychiatry 141, 496–502.



the prevalence of psychiatric disorders among people with intellectual disabilities: an analysis of...

Documents