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Microsimulation of Yoruba Fertility

M. G. SANTOW

National Blood Pressure Study, P.O. Box 691, Canberra City, A.C.T. 2601, Australia

Received I December 1977; revised 12 June I978

ABSTRACT

A microsimulation model is used to assess the extent to which the fertility of the

Yoruba of Western Nigeria may be affected by changes in the durations of lactation and

marital sexual abstinence. The simulations make no allowance for the compensatory use

of contraception. A series of preliminary simulations demonstrate the effect on fertility of

the length of the period of post partum non-susceptibility to conception, and an attempt is

made to duplicate the reported fertility of a large Ibadan survey (CAFNl). Input data are

then drawn from a number of recent Nigerian demographic surveys which enable the

separate simulation of the fertility of rural dwellers, poorer Ibadan women and richer

Ibadan women. The output indicates that, in the absence of contraception, urban fertility

is likely to exceed rural, and the fertility of richer urban women is likely to exceed that of

poorer urban women.

1. INTRODUCTION

The 1963 Nigerian census enumerated 11.3 million Yoruba, of whom

about 10 million inhabited the Western State (Lucas and Williams [13],

Orubuloye [IS]). About half now live in urban areas, with perhaps half of

these being concentrated in Lagos and Ibadan (Caldwell and Caldwell [6]),

although many urban residents live in traditional towns and are still

dependent on agriculture (Lucas and Williams [13]). In rural areas the crude

birth rate has been estimated at around 50 per 1000 and the crude death

rate at around 27 per 1000 (Lucas and Williams [13]).

The topic of this paper is the effect on Yoruba fertility of the partial

breakdown of the practice of marital sexual abstinence. Postpartum sexual

abstinence traditionally continued for at least as long as the mother

breastfed her child, and thus extended for two or three years. When

breastfeeding is prolonged for more than three or four months it is capable

of suppressing the return of ovulation and, therefore, the return of

menstruation. However, the average duration of amenorrhea is always

MATHEMATICAL BIOSCIENCES 42, 93-l 17 (1978) 93

BElsevier North-Holland, Inc., 1978

0025-5564/78/090087 + 25.SO2.25

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94

M G SANTOW

shorter than the average duration of lactation. The maximum contraceptive

effect of lactation is felt after about two years, beyond which point

prolonged lactation is no longer able to stave off the return of the menses

(see Buchanan [3]). Child spacing was therefore achieved through post-natal

sexual abstinence, as the first menstrual flow could be expected to occur

before sexual relations were resumed. Terminal abstinence was traditionally

adopted by married women in their forties, and affected fertility by truncat-

ing the reproductive span.

In some societies the optimistic belief is expressed that a lactating

woman cannot conceive (Santow [23]). In contrast, the Yoruba advocate

breastfeeding as a method not of preventing a new conception but of

ensuring the well-being of the child at the breast. Early weaning of a

Yoruba child will both leave him a prey to a variety of deficiency diseases

and synergistically lower his resistance to a host of other conditions. His

mother knows that such premature weaning may be induced by a new

conception too soon after his birth, while sexual abstinence prevents such a

conception and therefore protects her supply of milk. Thus Olusanya [17]

described the Yoruba belief that the milk of the lactating woman is made

harmful by intercourse, causing the child to fall ill and possibly die.

Moreover, a woman who became pregnant while she was still breastfeeding

was likely to be publicly criticized (Martin, Morley and Woodland [14]).

The short-term effect of abstinence is contraceptive in that it prevents a

conception. On the other hand, it should not be considered as contraceptive

in the long term, because its practice is unrelated to the number of children

already born to the family. Indeed, post partum abstinence should be

viewed as pro-natalist in intent, as it is the mothers method of maximizing

the number of her surviving children.

Caldwell and Caldwell [6] suggest that about two-thirds of abstinence

among Yoruba grandmothers is attributable to the strong belief that

grandmothers should not bear more children of their own. The demands of

such children for their mothers attention would jeopardize the strong link

between grandmother and grandchild. Such abstinence is plainly contracep-

tive in both intent and effect.

2. THE CASE FOR SIMULATION

The last Nigerian census was taken in 1963 and produced a sizeable

overcount (Lucas and Williams [ 131). More importantly, the establishment

Caldwell and Caldwell [6] write that the explanation for [the effect of a new

conception on the health of the unweaned child]

. .

s not always the correct one but the

incorrect explanation is often more vivid and probably more efficacious in enforcing

abstinence; amongst the Yoruba it is widely believed that the mans sperm actually enters

and poisons the milk which is being fed to the baby

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MICROSIMULATION OF YORUBA FERTILITY

95

of an effective system of vital registration in Nigeria is still in its infancy. AS

a result, fertility and mortality must be estimated from sample surveys,

which are subject to problems of omission and age misstatement. We shall

discuss these data defects in turn.

Respondents may fail to report a birth, particularly if, as is common, the

child is living with relatives rather than with his parents. Deaths, particu-

larly of very young children, may not be reported, and it may happen that

neither the birth nor the death of a child is reported if he died before he was

formally named. The failing memories of the more elderly (Brass [2]) may

be responsible for the omission of births and deaths by older women.

Deliberate omissions of births may occur in a situation similar to that

described by Lucas [l l] in which many Lagos respondents believed that

they would attract bad luck if they revealed the number of their children.

Soyinka [24] relates the response of an old woman, a character in a recent

African novel, to a question asking how many children she has:

Hush, we dont ask people how many children they have. It is not done. Children are not

goats or sheep or yams to be counted?

The second problem, that of age misstatement, may cause the misclassifi-

cation of reported vital events and therefore distort age-specific fertility and

mortality rates (see van de Walle [25]). All African demographic surveys

share the problem of trying to record the ages of people who do not know

their exact ages and are not fundamentally interested in knowing them

(van de Walle [26]). The most common source of error in all age reporting is

the overstatement of ages ending in certain preferred digits, with a corre-

sponding understatement of ages ending in other digits (Nagi, Stockwell

and Snavley [ 151). We shall return to this topic at a later stage of the paper.

Microsimulation provides a useful technique for measuring the effect on

fertility of rapid change amongst a group of intermediate variables, in this

case breastfeeding and post partum and terminal abstinence. For the sake

of clarity several areas of change were ignored in the simulations, namely,

infant and child mortality and contraceptive usage. Firstly, a recent

Nigerian study has demonstrated a very real differential between the infant

and child mortalities of two villages which differed principally in their

access to public health services (Orubuloye and Caldwell [19]). The im-

plementation of new health services is continuing and may be expected to

lead to an increase in the age-specific proportions of surviving children, as

the greatest effect of such services is the reduction of the exogenous

component of infant and child mortality. Secondly, although the level of

contraceptive use is low amongst the Yoruba of Ibadan, it has been

21n a Liberian study, Gay and Cole [8] observed that it is not proper to count aloud

even domestic animals lest some harm befall them.

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M. G. SANTOW

increasing over the last years (Caldwell and Caldwell [5]). The use of

microsimulation is indicated in this instance not only because of the

unreliability of the retrospective fertility data at our disposal, but as a

means by which to identify the effect on fertility of changes within one

group of linked intermediate variables in isolation from a number of others.

3. THE MODEL

The microsimulation model used in the following analysis is based on

those pioneered by Hyrenius and Adolfsson [lo] and Perrin and Sheps [20].

The time unit of the model is the so-called lunar month of 28 days, as this

is, on average, the longest period of time during which no more than one

conception can occur (Santow [23]). The reproductive span of each simu-

lated woman is defined by her ages at marriage and final sterility. Thus no

allowance is made for the possibility of pre-marital conception or marital

dissolution by divorce or the death of either spouse. As the Yoruba woman

generally remarries soon after her widowhood, the assumption that she

remains in some sort of sexual union for as long as she is fecund is not

unwarranted. The male partner does not appear explicitly in the model, but,

where appropriate, his contribution to fertility is represented by means of a

couple parameter. For example, the probability that a conception

terminates in a spontaneous abortion is a female parameter. On the other

hand, the probability that a conception occurred in the first place is a

couple parameter, because it is dependent on both of the marriage partners.

The first step in the simulation of each reproductive history is the

determination of the womans ages at marriage and final sterility. (Sterility

data are generally derived from the fertility non-experience of couples.)

The Monte Carlo technique is applied to the two relevant input distribu-

tions, and if the age at which sterility occurs does not exceed the age of

marriage, the woman exits from the simulation and the process is repeated

on the next woman.

Given that a positive number of reproductive cycles is initially allocated

to the woman under simulation, the Monte Carlo method is used to

determine whether she will conceive in the first cycle. If the first test is

unsuccessful it is repeated, and the number of such trials before a success

occurs gives the waiting time to conception in lunar months. The value of

the random number which identifies a conception also indicates whether the

conception is to terminate in a spontaneous abortion, a stillbirth or a live

birth. The lengths of the periods of gestation and of post partum

amenorrhea or sexual abstinence are determined from the input distribu-

tions specific to the pregnancy outcome.

The lunar-month counter is set to zero at marriage. After each Monte

Carlo test this counter is incremented either by one, in the case of an

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99

unprotected cycle in which no conception occurs, or by the duration of

non-susceptibility after a conception. This latter term is calculated as the

sum of the durations of pregnancy and post partum amenorrhea or sexual

abstinence. After each such incrementation the time counter is compared

with the predetermined reproductive span, and as soon as the counter

exceeds this span the reproductive history is terminated. Its fertility data are

stored, and the next reproductive history is simulated. When the sample is

complete the final aggregates are made. All the simulations are of 1000

women.

4. INPUT DATA

The input data are of two types. The first comprises biological input not

drawn specifically from the Yoruba but compatible, as far as is known, with

their condition. The second consists of data drawn from a number of recent

Nigerian demographic surveys. The following examples demonstrate how

the input data are inserted into the model.

We wish to determine whether a non-pregnant, fecund woman of a

particular age will conceive. In this example she is 22 years old, and thus the

probability of her conceiving is 0.20. A random number is selected, and

according to whether it is less than or exceeds 0.20, a conception is said to

occur or not. In the former case the random number also determines

whether the conception will terminate in an abortion, a stillbirth or a live

birth. A second random number is selected and is compared with the

appropriate cumulative frequency distribution from Table 1 in order to

TABLE 1

Percentage Distributions of Duration of Gestation and Post Partum Amenorrheaa

Months

Abortion

Gestation

Stillbirth

Live birth

Post partum amenorrhea

Abortion

Stillbirth

1

2

3

4

5

6

I

8

9

10

11

12

Mean

40

21

19

8

3

2

1

2.2

60

4

30

49

10

31

12

3

1

15 1

19

4

30

44

36 48

3

9.9 10.5 1.5 2.6

*Source: Clinical studies surveyed in Santow [23].

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M. G. SANTOW

determine the length of the pregnancy. If this pregnancy is to end in an

abortion, a random number of 0.50 gives a duration of two months.

Similarly, a random number of 0.50 gives a stillbirth gestation duration of

10 lunar months, and a live-birth gestation of 11 months.

4.1. BIOLOGICAL DA TA

The fecundability input is a simple age-dependent function increasing

linearly from a value of 0.13 at age 15 to 0.20 at age 20, remaining constant

until age 25 and then declining linearly to a value of 0.03 at age 42, at which

value it remains constant (Santow [23]). Spontaneous abortions account for

15 per cent of all conceptions, and stillbirths for a further 2 per cent.

The sterility input is an exponential function derived from Pittenger [21]

with a radix of 5 per cent of women sterile at age 17.5 and a median of 41

years.3 Whereas fecundability is determined solely as a function of age and

is the same for all fecund women of the same age, the point at which

sterility overtakes each woman is determined by the Monte Carlo method.

4.2. SURVEY DATA

Table 1 contains the three input distributions of gestation and those of

post partum amenorrhea which follow an abortion or a stillbirth. The

duration of post partum amenorrhea following a live birth depends on the

duration of lactation and will be discussed in connection with the breast-

feeding data obtained from the survey material.

Table 2 presents the characteristics of a number of Nigerian surveys (see

Okediji, Caldwell, Caldwell and Ware [16]). Data from all the surveys

indicated that virtually all Yoruba women are married by the age of 30, and

the mean age at marriage ranged from 18 to 22 years.4 Only 3 per cent of

the women from the entire NF2 sample had been married more than once,

although only 48 per cent of the sample were married monogamously. Of

the CAFNI women, 6.6 per cent were no longer in contact with their

husbands either through widowhood, separation or divorce, and the per-

centage of monogamous marriages was the same as in the much smaller

NF2 sample.

Figure 2 presents the population pyramid of CAFNl respondents aged

between 15 and 50 years, and clearly illustrates the problem of age misstate-

ment discussed earlier. Nagi, Stockwell and Snavley [ 151 considered that, in

age estimates, the over-reported digits are those which are multiples of the

divisors of the base of the number system, and that the extent of the

heaping is related to the magnitude of these divisors. These predictions are

3The choice of this radix was justified by the discovery that 5 per cent of the CAFNl

women (see Table 2) in the 45-59 age group were nulliparous.

the context of this work marriage is taken to mean any form of permanent sexual

union.

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MICROSIMULATION OF YORUBA FERTILITY 101

nicely fulfilled by the reported age statistics of the CAFNl women. There is

massive heaping on ages terminating in a 5 or a 0 (particularly between the

highly fecund ages of 20 and 30) although more women reported that they

were 25 than reported that they were 20. Preference was then given rather to

even terminal digits than odd. More specifically, the digits 2 and 8 were

more popular than 4 and 6, since these latter digits are overshadowed by the

highly preferred 5. Similarly, the digits 3 and 7 were less unpopular than 1

and 9.

The eccentricities of this population pyramid cast doubi not only on

reported age-specific fertility data but also on the reported distributions of

age at marriage. After smoothing, the means of the distributions obtained

from the NF2-1, NF2-2 and NF2-3 surveys were respectively 22, 18 and 22

years, and their ranges were (15,26), (14,21) and (16,28). The mean age at

marriage of the CAFN 1 distribution was 21 years, and the range was

(17,26).

Figure 3 presents graphically the Nigerian Family Project distributions

of the length of lactation, the corresponding length of amenorrhea and the

length of post partum abstinence. The duration of amenorrhea was not

sought in any questionnaire but was estimated using data from studies

which link lactation and post partum amenorrhea.5 The graphs facilitate

such comparisons as the percentage of women in each sample who are no

longer effectively sterile on their babys first birthday. Through abstinence

this percentage increases from 7 (NF2-1) to 21 (NF2-2) to 74 (NF2-3), while

through breastfeeding, the percentage increases from 46 (NF2-1) to 56

(NF2-2) to 100 (NF2-3). The figure shows quite clearly that for the richer

women (NF2-3) it is nursing, rather than abstinence, which provides the

greatest contraceptive protection for the first six months after confinement.

In other words, although the erosion of traditional practices has affected the

lengths of time both for which women are willing to breastfeed and for

which they are willing to abstain, it is the custom of post partum abstinence

which has been affected, in this case, the more drastically.

The lowest strip on the graph compares three pairs of distributions which

were obtained by asking women both how long post-natal abstinence

should continue, and how long they themselves had abstained after their

last live birth. Two pairs are drawn from the Fertility and Family Limita-

tion Survey (FFL) and the third, represented by unjoined points, from the

large Ibadan survey (CAFNl). In both sets of Ibadan data the should

abstain distribution lies fairly consistently above the did abstain distrib-

ution. One explanation for this is that the respondent does not anticipate

the death of an unweaned baby when she answers the should abstain

e Santow [23] for an account of such studies.

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104

M G SANTOW

I

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POPULATION LN

HUNDREDS

FIG 2 Population pyramid for CAFNI women aged 15 to 50 years

question, but that such a death prematurely curtails the period of post

partum abstinence and thus influences her response to the did abstain

question. With an annual infant mortality rate in Ibadan as high as 100 per

1000 live births (Santow [23]), it is likely that sufficiently many respondents

had recently experienced a child death for the did abstain distribution to

be depressed below the should abstain. [This does not explain why the

opposite trend is apparent in the Ekiti rural data (FFL-l).]

One feature common to all four sets of distributions is the irregularities

that occur at six-monthly intervals. For example, 32 per cent of NF2-2

women reported that they had stopped breastfeeding by the seventeenth

month post partum, but as many as 56 per cent reported that they had done

so by the eighteenth. Similarly, only 40 per cent had resumed sexual

relations by the twenty-second month, but 68 per cent had done so by their

childs second birthday. It seems likely that this heaping is not completely

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106

M. G. SANTOW

analogous with that observed in the reported age statistics, as the durations

of abstinence and nursing are often linked directly to the childs age. For

instance, a woman may resume sexual relations during a particular festival

at harvest time, recalling that she gave birth during this festival two years

before.6 The data presented in Figure 3 provide input distributions of post

partum amenorrhea and sexual abstinence specific to each survey, using a

time unit of half a year.

In the CAFNl sample, 55 per cent of women aged 40-44 years had

terminated all sexual relations, while 69 per cent had done so at ages 45-49,

and 83 per cent at ages 5&54. A further survey (NF3) was devoted

exclusively to female terminal abstinence and provided clear differentials in

the ages at which rural, poorer and richer Ibadan women became terminally

abstinent. By the age of 44 years the number of terminally abstinent women

comprised 53 per cent of the rural women (NF3-1 and NF3-2), 30 per cent

of the poorer Ibadan sample (NF3-3) and 30 per cent of the richer Ibadan

women (NF3-4). The average ages of commencement of terminal absti-

nence varied only from 36.4 years @F3-3) to 40.7 years (NF3-l), but the

age distributions of entrance into the abstinent state showed considerable

variation.

The NF3-1 and NF3-2 data were aggregated to provide an age distribu-

tion of entry into the terminally abstinent state compatible with NF2-1

data. Similarly, the NF3-3 terminal abstinence data were applied to the

NF2-2 simulation, and the NF3-4 data to NF2-3. Single-year distributions

were constructed, and as the NF3 sample included no women older than 44

years, the proportions of terminally abstinent women at higher ages were

obtained by linear extrapolation, with all women terminally abstinent by

the age of 50.

5. FERTILITY AND THE PERIOD OF POST PARTUM NON-SUS-

CEPTIBILITY TO CONCEPTION

A series of simulations were performed to test the effect on fertility of

increasing the duration of post partum non-susceptibility to conception

from two lunar months to three years. A constant marriage age of 17 years

was employed, and the remaining input data were used as described in the

previous section.

Table 3 presents the means, and Fig. 4 the distributions, of live births

corresponding to seven distinct periods of post partum nonsusceptibility.

contrast, the preliminary analysis by R. Lesthaeghe and H. Page of the Lagos

Parity Study data on breastfeeding, post partum amenorrhea and abstinence detected

considerable heaping in the retrospectively obtained distributions, but not in the prospec-

tive distributions. This suggests that in Lagos (which is a modem city by comparison with

Ibadan) such heaping is due more to classical misstatement than to a true link between the

childs age and the durations of breastfeeding and abstinence.

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107

TABLE 3

Average Parity by Duration of Post Partum Non-susceptibility to Conception

Non-susceptibility

2

5 9

13

18 25 35

(lunar months)

Average parity

12.94 11.52 9.92

8.66

7.55 6.56 5.32

Width of 95

confidence

intervala

0.54

0.47 0.42 0.38

0.31 0.27 0.23

aThe width of the 95 confidence interval is obtained by dividing twice the sample

standard deviation by the square root of the sample size, and multiplying the result by

1.96.

FIG 4 Distributions of live births by fixed duration of abstinence in lunar months.

Progressively greater increments in the duration of non-susceptibility need

to be selected in order to demonstrate a fertility differential of at least one

live birth.

A simple formula demonstrates the approximate relationship between

fertility and the length of the period of post partum non-susceptibility. If

we set

a as the sum of

the waiting time to conception and

the gestation period,

k as the length of non-susceptibility,

R as the reproductive span, and

F as the average fertility,

This trivial example ignores such factors as fetal loss.

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108

then

M G SANTOW

A further set of simulations tested the effect on fertility of the use of a

distribution of non-susceptibility rather than a constant duration. Neither

average fertility, nor the width of the corresponding 95 per cent confidence

interval, was affected by the change in the form of the non-susceptibility

input. As the duration of non-susceptibility increases it tends to dominate

the other factors which contribute to the determination of ultimate fertility.

This occurs because the proportion of the time expended in producing a live

birth which is contributed by the non-susceptible period is, in terms of the

analytic model,

&-1-s.

(2)

Consequently it is this factor, rather than the use of a constant duration of

post partum non-susceptibility, which is responsible for the shrinking of the

confidence intervals.

6.

PRELIMINARY TESTING-CAFN 1 DATA

The next task assigned to the model was to attempt to simulate the

fertility of the CAFNl sample. This survey sought no information on

breastfeeding, but we have seen that the proportion of women still breast-

feeding exceeds the proportion still abstaining only during the first six

months post partum of the richer Ibadan women (NF2-3). Caldwell and

Caldwell [6] noted that

in rural areas, the period of abstinence is shorter than the period of lactation even now in

only about one case in twenty where the child survives, and further investigation usually

reveals that such atypical behaviour is exhibited only by persons who have broken

substantially with the traditional culture.

Consequently, in all but the most exceptional cases it is abstinence, rather

than post partum amenorrhea, which provides the greatest post partum

protection against conception, as the duration of lactation exceeds that of

post partum amenorrhea.

Two simulations were performed which incorporated the CAFNl did

abstain and should abstain distributions, and the CAFNl estimated

distribution of female age at marriage. The 95 per cent confidence intervals

around the average completed fertilities of these runs were, respectively,

(5.14,5.42) and (5.25,5.54), and their means were 5.28 and 5.40 live births.

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109

An attempt was then made to estimate completed fertility directly from

the 400-odd CAFNl respondents in the 45-59 age group who stated that

they had never used any contraception.* This condition removed nearly 10

per cent of the women in this age group, but was deemed necessary because

the only artificial constraints which the simulation imposed on fertility

were the age at marriage and the length of post-natal abstinence. Despite

this adjustment, the fertility estimate is still a crude one because it embodies

the assumptions that the respondents have completed their childbearing,

that they have reported their reproductive histories with complete accuracy,

and also that the fertility of these fifteen birth cohorts remained constant.

The fertility estimated in this way was 5.15 live births. Slightly over half

of these women were in the 50-59 age group, and their average fertility was

5.39 live births. This differential may be caused by the falsity of the

assumption that women in the 45549 age group have completed their

childbearing, or by a decline in cohort fertility.

It is encouraging to find such agreement between the simulated and

reported average fertilities. However, one is confronted with an obstacle

when one tries to make sense of the recorded age-specific fertility data,

because of the unreliability of the age data on which they are based (see

Fig. 2). As an alternative approach, therefore, the distribution of live births

was estimated from the testimony of the CAFNl women in the 4549 age

group.

This distribution is compared with the two simulated distributions in Fig.

5. The modal frequency of the reported distribution is lower than those of

the simulated distributions, and the range is slightly greater. Moreover, the

reported distribution is characterized by jagged irregularities caused by the

tendency to report an even number of children. Except for the percentage

of women who reported nine births, the percentage of women who reported

an odd number of births is less than the percentages who reported either

one more or one less than this number.

A similar preference for even parities was demonstrated by the responses

in another survey to a question on desired family size (Santow [23]). It

appears that the Yoruba find it easier to conceptualize a family of an even

number of children than an odd number, and that older women, at least, do

not perceive a question on achieved parity as being obviously less theoreti-

cal than a question on desired parity. A simple analytic treatment demon-

strates the effect of this even preference on the reported completed fertilities

*Contraception here includes the use of charms or medicines sold by a native doctor,

rhythm, withdrawal, condoms, jellies, creams, sQline pessaries, douching, diaphragm,

foam, internal ring, orals, I.U.D., the sterilization of either partner and abortion. See

Caldwell and Caldwell [5].

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M. G. SANTOW

FIG. 5.

Distribution

of live births of real and simulated populations with long periods

of post partum sexual abstinence.

of fertile women. If we set

i as the number of births where i = 1,2,..,n for n=2k+ 1,

4

as the number of women with i births,

n, as the number of women with i births who report i 1 births,

n,- as the number of women with i births who report i - 1 births, then

the average fertility is

We assume that ni+ = nip =0 when i is even, and that the extent of

individual misreporting does not exceed one birth. Then the reported mean

fertility, FR,

can be expressed as

4)

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Consequently, FR F when

111

(5)

Thus, if the total number of odd-parity women who report one extra birth

equals the number who report one fewer, that is, if the direction of the

distortion is random, then the average reported fertility will still represent

the average achieved fertility.

A stronger condition for equality is that

nZ+

I = nzi+3

for i=O, l,..,k- 1.

(6)

7. BREASTFEEDING, ABSTINENCE AND FERTILITY-NF2

DATA

The sizes of the NF2 sub-samples preclude the direct estimation even of

average completed fertility. On the other hand, the distributions of post

partum non-susceptibility to conception are no more irregular than that

derived from the CAFNl data. Indeed, this latter distribution is similar to

the abstinence distribution obtained from the poorer urban sample (NF2-2).

TABLE 4

Age group

Simulated Age-Specific Fertility per loo0 Women*

NF2-1

NF2-2 NF2-3

Lactation Abstinence Lactation Abstinence Lactation Abstinence

10-14

lSl9

20-24

25-29

3 34

35-39

4549

5 54

18 20

455

376

912 814

247 271

1369 1010

1990 1521

1610 1566

1862 1258 1902 1421 2515 2319

1598 1105

1618

1293

2143 1952

1122

810

1145

897

1433 1394

458 362

426

394

527

474

82 65

81 67 105

97

4

3

6 7

7

6

Total

6950

4989 8098

6434 8587

8079

Mean post partum

14.6

29.9 14.0

23.6 6.7

8.4

infecundable period

Mean marriage

age

22

22 18

18 22

22

Modalnumberof 7 6 9 7 10 8

live births

Using NF2 data on distributions of lactation and abstinence.

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112

M G SANTOW

Table 4 presents the age-specific fertilities of the six simulations of the

NF2 sub-samples which incorporate independently the distributions of

lactation and abstinence as depicted in Fig. 3. A number of inferences may

be drawn from the table. The mean lengths of lactation amenorrhea are

almost the same for the rural and poorer groups (NF2-1 and NF2-2

respectively), but the women of the latter group marry about four years

earlier than those of the former. This earlier marriage age adds, on average,

over one live birth per woman to the completed fertility of the first group.

Similarly, the rural and richer women (NF2-1 and NF2-3) marry, on

average, at the same age, but the richer women are amenorrheic for less

than half the period which is usual amongst the rural women. This has an

even greater effect on fertility than the first comparison, as the richer

women each produce over one and a half more live births, on average, than

the rural women.

When one examines the simulations which incorporate post partum

sexual abstinence, one finds that the marriage differential between the rural

and poorer samples, combined with the shorter period of abstinence of the

latter, produces an extra one and a half live births. Moreover, the richer

women each produce about three mole babies than the rural ones because

they abstain for a much shorter period. In comparative terms, this fertility

increase is 62 per cent of the fertility of the rural women.

Figure 3 reveals that, amongst the NF2-1 and NF2-2 women, the

importance of the duration of breastfeeding is everywhere negated by the

duration of sexual abstinence. However, in the NF2-3 sample it is the

duration of nursing which is the dominant factor for the first few months

post partum. Having demonstrated the individual effects of the distributions

of lactation and sexual abstinence on the fertility of the NF2 sub-samples,

we next aggregate these distributions to produce just one distribution of

post partum non-susceptibility for each sub-sample. For the NF2-1 and

NF2-2 samples, therefore, this new distribution is identical with the old

abstinence distribution, but a truly new distribution was constructed for the

NF2-3 sample from Fig. 3 by taking successively the minimum percentage

of the distributions of abstinence and lactation amenorrhea.

Figure 6 presents the live birth distributions of these three new NF2

simulations. There is a very clear shift from the steepest distribution of the

NF2-1 sample to the flatter one of NF2-3, and a corresponding increase in

the range of possible completed family sizes.

Table 5 compares the age-specific fertilities of the runs shown in Fig. 6

with those of a set of new simulations which incorporate terminal absti-

nence. As the input data provide for only 1 per cent of women to be

terminally abstinent by the age of 34, one looks for the first effects of such

abstinence in the fertility of the 35-39-year-olds. Indeed, there is a drop of

about 10 per cent in each of the three runs in this age group, but in the

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113

FIG. 6. Distribution of live births of the three Yoruba simulations.

40-44 age group the declines are respectively 47 per cent, 30 per cent and 32

per cent.

The variations in the fertility decreases exemplify once again the dif-

ferential erosion of a traditional practice, for although the fertility of the

urban women has declined, the extent of the decline is not as great as that

of the rural women. The mean completed fertility of each run is signifi-

cantly lower at the 0.05 level than that of its parent run which does not

TABLE 5

Simulated Age-Specific Fertility of NF2 Sub-samples Showing the Effect of Terminal Abstinence

Original runs

NF3 Terminal abstinence

Age group

NF2-1 NF2-2

NF2-3 NF2-1 NF2-2

NF2-3

10-14 0 20 0

0 17 0

15-19 376 814 251

369 798 228

2 24 1010 1521 1496

973 1486 1456

25-29 1258 1421 2188

1259 1438 2186

30-34 1105 1293 1923

1139 1241 1875

35-39 810 897 1274

738 798 1146

362 394 520 192 274 352

4549 65 67 83

25 16 39

5G54 3 7 7

0 0 0

Total 4989 6434 7742

4695 6068 7282

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114

M. G. SANTOW

allow for terminal abstinence, with the falls in mean fertility increasing from

about 0.3 live births for the rural women to 0.4 for the poorer Ibadan

women and to 0.5 for the richer Ibadan women.

8. CONCLUSIONS

The demonstration of the impact on fertility of the length of the period

of post partum non-susceptibility to conception, and the comparison of

simulated and reported CAFNI fertility data, encouraged an attempt to

simulate the fertility of the three groups of Yoruba women, namely, rural,

poorer ibadan and richer Ibadan. The preliminary simulations were refined

by the inclusion of patterns of terminal abstinence appropriate to each

sub-sample.

We may regard the three final simulations as static representations of the

fertility experience of a population at different stages in a process of

increasing Westernization accompanied by a weakening of the force of

traditional practices. Viewed in this way, the simulations possess dramatic

implications: the rural fertility of 4.7 live births increases to the poorer

urban fertility of 6.1 iive births, to the richer urban fertility of 7.3 live births.

In centers other than Ibadan one would expect these figures to be slightly

different because of different patterns of marriage, lactation and sexual

abstinence, but the overall conclusion would not change. As women move

from the villages and towns into the city, they can be expected to produce

nearly one and a half more babies than their sisters who stayed at home. As

the financial situation of these urban women improves they can be expected

to produce, on average, slightly more than one additional child, and this

fertility increase would be even greater but for the fact that these women

marry about four years later than their poorer urban counterparts.

In her study of a central Javanese village Hull [9] found a positive

correlation between the average numbers of ever born and surviving

children, and income. Women in the 25-44 age group were differentiated by

two patterns of abstinence according to income, with lower-income women

abstaining for about five months longer than upper-income women. There

was no difference in abstinence levels by income for women in the 15-24

age group, although the periods of stated abstinence were about six months

shorter, at ten months, than those reported by the lower income women in

the 25-34 age group. However, in contrast with this trend, Hull [9] noted

that a later age at marriage of women with higher levels of schooling was

seen to affect cumulative fertility averages of current 2624 year olds.

That urbanization and economic development lead to a reassessment of

personal goals (or even to the first such assessment) and hence to a

gFor example, Adegbola, Page and Lesthaeghe (1) discovered much shorter periods of

breastfeeding and post partum abstinence in Lagos than Ibadan.

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115

reduction in fertility, is a theme which permeates much demographic

work.O If one manages to free ones mind from this preconception, one

need not be puzzled by the existence of a positive relation between eco-

nomic class and urbanization on the one hand and fertility on the other. In

the Nigerian case, increasing urbanization and economic development are

facets of a general process of change which is also affecting the adherence

to customs whose effect is the spacing of births and whose intent is the

maximizing of surviving fertility. One is led to the conclusion that the gap

left by the breakdown of traditional methods of spacing births is ready to

be filled by the efficient use of contraception. In the words of Dow [7],

as such [African pronatal spacing] intentions have not changed greatly over time, con-

traception would represent merely a substitution of means in the pursuit of relatively

constant ends or values.

One might predict that the use of contraception for spacing purposes might

lead to its use for limiting purposes.

An examination of studies on contemporary Nigeria recalls us from such

realms of speculation. The shortening of the abstinence period by means of

family planning was deemed a good thing only by 31 per cent of rural

women (NF2-1) and 16 per cent of poorer urban women (NF2-2), but by 80

per cent of richer urban women (NF2-3). Wares [27] analysis of Ibadan

survey data found that only 16 per cent of all Yoruba women personally

wish to have four or fewer children. Moreover,

universal acceptance of the four-child family would not drastically reduce population

growth rates, but it would represent a significant step towards the modernization of family

size ideals.

On the positive side, however, the proportion of women who have never

used family planning fell from 91 per cent of the rural women and 94 per

cent of the poorer urban women, to 30 per cent of the richer urban women.

While only 16 per cent of CAFN 1 women have ever used modem con-

traception, Caldwell and Caldwell [5] report not only a low dropout rate

amongst users, but also that the level of contraceptive usage has doubled

every four years during the previous twenty. The simulations indicate the

magnitude of the gap left by the breakdown of marital sexual abstinence

which needs to be filled by the use of modem contraceptives if Yoruba

fertility is not to undergo a dramatic increase.

I am grateful to Professor J. C. Caldwell of the Demography Department,

Australi an National University, and to the late Professor F . 0. Okedgi of the

OSee.Robinson [22] and Caldwell [4].

Lucas and Ukaegbu [12] cite a comparable figure of 3 per cent of the rural Ngwa

Ibo.

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116

M. G. SANTOW

Soci ology Department , Uni versig of I badan, for permi ssi on t o use data from

t he N i geri an surveys of t he Changi ng A fri can Famil y project . I am grateful t o

t he ormer for permi ssi on t o use data rom t he Ni geri an Fami ly Proj ect surveys,

and to M r. I . 0. O rubuloy e, al so of t hi s department , f or permi ssion t o quote

data from his Fertility and Fami Limitation surveys.

REFERENCES

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

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