3'ournal of Epidemiology and Community Health 1996;50:245-251

A prospective study of social networks inrelation to total mortality and cardiovasculardisease in men in the USA

Ichiro Kawachi, Graham A Colditz, Alberto Ascherio, Eric B Rimm,Edward Giovannucci, Meir J Stampfer, Walter C Willett

AbstractStudy objective - Previous studies haveestablished a relationship between lowlevels of social networks and total mor-tality, but few have examined cause spe-cific mortality or disease incidence. Thisstudy aimed to examine prospectively therelationships between social networks andtotal and cause specific mortality, as wellas cardiovascular disease incidence.Design - This was a four year follow upstudy in an ongoing cohort of men, forwhom information on social networks wascollected at baseline. The main outcomemeasures were total mortality, further ca-tegorised into deaths from cardiovasculardisease (stroke and coronary heart dis-ease), total cancer, accidents/suicides, andall other causes; as well as stroke and cor-onary heart disease incidence.Participants - Altogether 32 624 US malehealth professionals aged 42 to 77 yearsin 1988, who were free of coronary heartdisease, stroke, and cancer at baseline.Results - A total of 511 deaths occurredduring 122 911 person years of follow up.Compared with men with the highest levelof social networks, socially isolated men(not married, fewer than six friends orrelatives, no membership in church orcommunity groups) were at increased riskfor cardiovascular disease mortality (ageadjusted relative risk, 1*90; 95% CI 1-07,3X37) and deaths from accidents andsuicides (age adjusted relative risk 2-22;95% CI 076, 6-47). No excess risks werefound for other causes of death. Sociallyisolated men were also at increased riskof stroke incidence (relative risk, 2-21;95% CI, 1-12, 4.35), but not incidence ofnon-fatal myocardial infarction.Conclusions - Social networks were as-sociated with lower total mortality by re-ducing deaths from cardiovascular diseaseand accidents/suicides. Strong social net-works were associated with reduced in-cidence of stroke, though not of coronaryheart disease. However, social networksmay assist in prolonging the survival ofmen with established coronary heart dis-ease.

(J Epidemiol Community Health 1996;50:245-251)

The association between social realtionshipsand health was originally described by Durk-

heim,1 who reported an increased risk ofsuicide among socially isolated individuals.Subsequent epidemiological research has es-tablished that social networks predict not onlythe risk of suicide, but all-cause mortality.2 Todate, eight prospective epidemiological studieshave reported an increased risk of total mor-tality in socially isolated individuals.`'0 Despitethe wealth of evidence on social networks andtotal mortality, some important questions re-main unanswered.

Firstly, it is not clear whether the increasedrisk of total mortality reflects a generalisedsusceptibility to illnesses among socially isol-ated individuals,"1 or whether such persons areat especially high risk of mortality from specifictypes of illness. There is some evidence thatsocial isolation is associated with an increasedrisk of cardiovascular disease mortality6'2 andtotal cancer mortality." However, virtually allof the prospective studies to date have reporteddata on total, but not cause specific, mortality.Secondly, it is not known where along thespectrum of disease social networks exert theirimpact. The major prospective studies of socialnetworks have examined mortality, but not dis-ease incidence. Thus it is not clear whethersocial networks influence disease incidence,recovery, or case fatality. 'To address some of these unanswered ques-

tions about social networks, we prospectivelystudied the associations between social net-works and subsequent disease incidence andcause specific mortality.

MethodsThe health professionals follow-up study is alongitudinal study of risk factors for cardio-vascular disease and cancer among 51 529 USmen aged 40 to 75 years in 1986. The studypopulation consists of 29 683 dentists, 10 098veterinarians, 4185 pharmacists, 3745 op-tometrists, 2218 osteopathic physicians, and1600 podiatrists. The study began in 1986,when cohort members completed a mailedquestionnaire on heart disease and cancer riskfactors, medical history, and diet. Follow upquestionnaires were sent in 1990 and 1992 toupdate this information.

ASSESSMENT OF SOCIAL NETWORKS AND OTHEREXPOSURESAll participants in the study were asked to'complete the Berkman-Syme social networks

Department ofNutritionA AscherioW C Willett

Department ofEpidemiologyE B RimmM J Stampfer

Department of Healthand Social BehaviorI Kawachi

Harvard School ofPublic Health andChanning Laboratory,Boston, USADepartment ofMedicineG A ColditzE Giovannucci

Brigham and Women'sHospital and HarvardMedical School,Boston, USA

Correspondence to:Assistant ProfessorI Kawachi,Department of Healthand Social Behavior,Harvard School ofPublic Health,677 Huntington Avenue,Boston MA 02115, USA.Accepted for publicationJanuary 1996

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index as part of the 1988 mailed questionnaire.The social networks index was originally de-veloped in the Alameda County study,3 and isa composite measure of four types of socialconnection: marital status (married versus notmarried); sociability (frequency and number ofcontacts with extended family and close friends,rated on a scale of 1 to 5, with high valuesassociated with many contacts and low valueswith few); church group membership (yes ver-sus no); and membership in other communityorganisations (yes versus none). Responses tothe index are categorised into four levels ofsocial connection: low networks (level I) - forexample, characterised by individuals with lowintimate contacts (not married, fewer than sixfriends or relatives), and no membership ineither church or community groups; mediumnetworks (level II); medium-high networks(level III); and high networks (level IV). Fur-ther details of the construction of the socialnetworks index are described elsewhere.'5

In addition to the assessment of social net-works, we obtained information from the base-line and follow up questionnaires on theparticipants' medical history, current smokinghabits, body mass index, levels of physical ac-tivity, alcohol intake, parental history of myo-cardial infarction (including the age of eachparent at the first event), and participants' selfreported history of diabetes mellitus, hy-pertension, and hypercholesterolaemia. Theaccuracy of a self reported diagnosis of hy-pertension was confirmed and reported in avalidation study among 100 cohort members.'6

ASSESSMENT OF TOTAL AND CAUSE SPECIFICMORTALITYThe primary end points in our study compriseddeaths from all causes that occurred betweenthe return of the 1988 questionnaire andJanuary 31, 1992. Most deaths were reportedby next of kin, work associates, or postalauthorities. In addition, the mortality sur-veillance included systematic searches of thevital records using the national death index todiscover deaths among participants who werepersistent non-responders to the questionnairemailings. We estimate that more than 98% ofthe deaths are ascertained by these methods.'7

Physicians reviewed death certificates andhospital or pathology reports to classify in-dividual causes of death. As well as analysingtotal mortality, the deaths were grouped intofour broad categories: cardiovascular diseases(coronary heart disease, ICD 9th revision codes410 to 414, and 798; and stroke ICD codes430 to 438); total cancers (ICD 9th revisioncodes 140 to 209); external causes of injury(all ICD "E" codes, which included deathsfrom accident, poisoning, suicide, and othertrauma); and all remaining causes of death.

ASSESSMENT OF CARDIOVASCULAR DISEASEINCIDENCEWe also examined all incident cases ofcoronaryheart disease and stroke occurring between thereturn of the 1988 questionnaire and January

31, 1992. We wrote to all participants reportingan incident myocardial infarction or stroke onthe 1990 or 1992 questionnaires to requestpermission to review their medical records.A definite myocardial infarction was classifiedaccording to the criteria of the World HealthOrganization,'8 and required symptoms pluseither typical electrocardiographic changes orhigh levels of cardiac enzymes. A definite strokewas classified as confirmed if the criteria of thenational survey of stroke were met. 9 Incidentcases of non-fatal myocardial infarction andnon-fatal stroke were classified as "probable"if the hospital records could not be obtainedbut the event required admission to hospitaland the diagnosis was corroborated by sup-plementary correspondence or telephone in-terview.

Fatal coronary heart disease was made up ofcases of fatal myocadial infarction confirmedby hospital records or autopsy reports; cases ofsudden cardiac death (defined below); plusother deaths from coronary heart disease asdetermined from death certificates, if evidenceofprevious coronary heart disease was available(either from hospital records or interviews withthe next of kin) and this was the underlyingand most plausible cause of death. In no in-stance did we use death certificate classificationalone to categorise a death as due to coronaryheart disease. Sudden cardiac death was de-fined as death occurring within 1 hour of theonset of symptoms in a man with no previousserious illness, for which no other plausiblecause of death - other than coronary disease -was reported.Death due to stroke was ascertained by phys-

ician review of hospital records and autopsyreports. In this study, 92% of the non-fatalmyocardial infarction and fatal coronary heartdisease, and 87% of the non-fatal and fatalstrokes were classified as definite events.

STUDY POPULATION AND DATA ANALYSISBecause the onset of major illness could itselfinfluence an individual's social networks, weexcluded from analyses all members of thecohort who reported a diagnosis of myocardialinfarction, stroke, or cancer (except non-melanoma skin cancer) before the return of the1988 questionnaire (n = 6115 men). We cannotexclude the possibility that some men had un-diagnosed illnesses at baseline. On the otherhand, there is no reason to believe that anundiagnosed illness would have affected anindividual's social network. We additionally ex-cluded 12 790 men who after three mailingsdid not complete the main 1988 questionnairebut instead answered a shorter questionnairethat did not include the Berkman-Syme socialnetworks index. This left a total of 32 624 menavailable for follow up. After repeated mailingsto non-respondents, including the use of cer-tified mail, we obtained a 96% response rateto the 1990 questionnaire, and a 94% responserate to the 1992 questionnaire.

In the analyses, each participant ac-cumulated person-months of follow up fromthe date of return of the 1988 questionnaire to

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Table 1 Age standardised distribution of risk factors in relation to level of social network index

Berkman-Syme social network index

Risk factor IV (high) III II I (low)

No (%) 16807 (51 5) 6216 (19 1) 7 706 (23 6) 1 895 (5-8)Smoking (%) 7-3 9.1 9 5 12.1Hypertension (%) 191 20 4 20 5 20.1Diabetes (%) 2 7 2-5 2 8 2 8High cholesterol (%) 168 180 174 16 4Body mass index (kg/M2) 25-1 25-0 24 9 24 9Parental history of MI before age 60 (%) 117 125 131 12-0Alcohol intake > 15 g/d (%) 11-4 16-0 17-0 19 6Physical activity (MET-h/week) 20-0 19 5 19 5 20 1Blood pressure check in in past 2 years (%) 862 83-2 82 7 79 3Cholesterol check in past 2 years (%) 70 4 68-6 65 8 62 1Screening physical exam in past 2 years (%) 668 64-0 60 3 57 1

Table 2 Age adjusted and multivariate relative risks of total and cause specific mortality according to level of socialnetwork index 1988-92

Berkman-Syme social network index

IV high III II I (low) Test for trend, pTotal mortalityCases 234 101 133 43Age adjusted RR 1 00 1 11 1 22 1-57

(0-88, 1-41) (0-99, 1 51) (1-14, 2 17) 0 004Multivariate RR* 1-00 1-06 1 13 1 38

(0-84, 1 35) (0 91, 1-40) (0-99, 1-93) 0 06Cardiovascular disease (stroke and coronary heart disease)Cases 63 31 45 14Age adjusted RR 1-00 1-28 1-53 1-90

(083, 1 96) (1 05, 2-24) (1-07, 3 37) 0006Multivariate RR 1-00 1 21 1 44 1-76

(0 79, 1-87) (0-98, 2-13) (0 97, 3 16) 0-02Total cancerCases 100 41 39 12Age adjusted RR 1-00 1-05 0-84 1-02

(0-73, 1-52) (0-58. 1 21) (0-56, 1 84) 0-55Multivariate RR 1-00 1-03 0-78 0-87

(0 71, 1 48) (0-54, 1-13) (0-47, 1 60) 0 25Accidents and suicidesCases 16 10 18 4Age adjusted RR 1 00 1 68 2-48 2-22

(0 77, 3 66) (1 29, 4-75) (0-76, 6 47) 0-008Multivariate RR 1 00 1 62 2-35 1 99

(0 73, 3 59) (1-19, 4-62) (0 66, 5-99) 0-02Other causesCases 55 19 31 13Age adjusted RR 1 00 0 88 1 23 1-97

(0 52, 1-48) (0 79, 1-89) (1-09, 3-58) 0 06Multivariate RR 1-00 0 78 1 09 1 48

(0-46, 1-33) (0 70, 1 70) (0-79, 2 76) 0 32

*Multivariate relative risks adjusted for age (5 year age categories), time period (1988-90; 1990-92), smoking status (never,former, and current in categories of 1 to 14, 15 to 24, and 25 or more cigarettes per day), history of hypertension, diabetesmellitus, and hypercholesterolemia, diagnosis of angina pectoris, deciles of body mass index, parental history of myocardialinfarction before age 60 (yes/no), daily alcohol intake (0, 0-414-9, 5-0-14-9, 150-24-9, 250-49-9, or 50 or more g/day), andtertiles of physical activity.

January 31, 1992 (or, for those who died ordeveloped cardiovascular disease, up to thedate of the event). Proportional hazards modelswere used to adjust for age (in 5 year agecategories), cigarette smoking (categorized ascurrent smokers of 1 to 14, 15 to 24, or 25+cigarettes per day, past, or never smokers),alcohol intake (00 g/day, 0-01-4-9, 5-0-14-9,15-0-24-9, 25-0-49-9, and 50-0 or more g/day), body mass index (deciles), history ofhypertension, diabetes mellitus, hyper-cholesterolaemia, angina pectoris, family his-tory of myocardial infarction before age 60,and physical activity (tertiles). When ap-propriate, we performed the Mantel test forlinear trend across levels of social networks,and reported the two tailed p values.20

ResultsCHARACTERISTICS OF THE STUDY POPULATIONAltogether 5-8% of the study population weresocially isolated (level I of the social networksindex), while 23-6% were in level II (medium

networks), 19-1% were in level III (medium-high networks), and 51-5% were socially wellintegrated (level IV). We compared the agestandardised distributions of health behavioursand risk factors across the levels of social net-works (table 1). Socially isolated individualswere more likely to be current smokers and todrink more than 15 g/day alcohol (p for lineartrend across categories of social networks<0-0001, for both smoking and drinking). So-cially isolated men were also less likely to haveundergone a blood pressure check, a serumcholesterol check, or a physical examinationduring the past two years for screening pur-poses. Other health related habits were dis-tributed similarly across the groups.

TOTAL MORTALITYDuring the four years of follow up, 511 deathsoccurred in 122911 person-years. One hun-dred and fifty three individuals died of cardio-vascular diseases (135 coronary heart diseasedeaths and 18 stroke deaths); 192 of total

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Table 3 Age adjusted and multivariate relative risks of total, non-fatal, andfatal stroke incidence according to level ofsocial network index 1988-92

Berkman-Syme social network index

IV high III II I (low) Test for trend, p

Total strokeCases 40 21 33 10Age adjusted RR 1-00 1-39 1-82 2 21

(0-82, 2-37) (1-15, 2-87) (1-12, 4 35) 0-002Multivariate RR* 1-00 1 31 1-72 2 02

(0 77, 2 23) (1-08, 2 73) (1-00, 4 08) 0-008Fatal stroketCases 5 0 6 2Age adjusted RR 1 00 - 2-64 3-64

(0-84, 8 27) (0 78, 16-9) 0-04Non fatal strokeCases 35 21 27 8Age adjusted RR 1-00 1-55 1 65 1 96

(090, 266) (100, 271) (092, 4-17) 0-02Multivariate RR 1 00 1-50 1 61 1-86

(0-87, 2 60) (0-97, 2 67) (0-85, 4 06) 0 03

*Multivariate relative risks adjusted for age (5 year age categories), time period (1988-90; 1990-92), smoking status (never,former, and current in categories of 1 to 14, 15 to 24, and 25 or more cigarettes per day), history of hypertension, diabetesmellitus, and hypercholesterolemia, diagnosis of angina pectoris, deciles of body mass index, parental history of myocardialinfarction before age 60 (yes/no), daily alcohol intake (0, 0-014-9, 5-0-14-9, 15-0-24-9, 250-49 9, or 50 or more g/day), andtertiles of physical activity.t Too few cases to perform multivariate analysis.

cancer; 48 of accidents and suicide; and 118of other causes.

Compared with the group with the mostsocial connections (level IV), socially isolatedindividuals (level I) were at 1-5 times the age

adjusted risk of total mortality (p for trendacross categories of social networks index=0-004) (table 2). Adjustment for a broad range

ofrisk factors in multivariate analysis somewhatattenuated the association. The following com-ponents of the social networks index predictedtotal mortality (data not shown in tables): notbeing married (relative risk, 1-41; 95% CI 1-07,1-87); not belonging to a church group (relativerisk, 1I42; 95% CI 1 19, 1-70); and the absenceof close relatives (relative risk, 1-34; 95% CI1.01, 1-78).

CAUSE SPECIFIC MORTALITY

Socially isolated individuals were at increasedrisk of cardiovascular diseases (p for trend=0 006), and accidents and suicides (p fortrend=0-008) though not total cancer (p fortrend = 0 55). Of the individual components ofthe social networks index, being unmarriedwas the strongest predictor of cardiovasculardisease mortality (relative risk, 1-44; 95% CI0.88, 2-38). We analysed stroke mortality sep-arately from coronary heart disease mortality(data not shown in tables). Compared with thegroup with the highest level of social ties (levelIV), the age adjusted relative risk of strokemortality was 1 31 (95% CI 0-24, 7 23) amongmen in the medium-high group (level III),4 90 (95% CI 1 70, 14 16) among men in themedium group (level II), and 6-59 (95% CI1-78, 24-38) among men in the low group (levelI) (p for trend, 0-0008, based on 18 deaths).By contrast, the association between social net-

works and coronary mortality was weaker: rel-ative to the group with the highest level ofsocial ties (level IV), the age adjusted relativerisk of coronary heart disease mortality was

1-28 (95% CI 0-82, 1-99) among men in themedium-high group (level III), 1-31 (95% CI0.86, 1-98) among men in the medium group

(level II), and 1-59 (95% CI 0-84, 3.02) amongmen in the low group (level I) (p for trend,0-09, based on 135 deaths).Three components of the social networks

index strongly predicted mortality from ac-cidents and suicides: not being married (relativerisk, 2-92; 95% CI 1-50, 5-67); not belongingto a church group (relative risk 1-99; 95% CI1-14, 3 47); and the absence of close relatives(relative risk, 3-83; 95% CI 1-97. 7 47). Twentycases of suicide occurred during the four yearstudy period. The age adjusted relative risk ofsuicide among individuals in the most sociallyisolated group (level I) was 3-22 (95% CI 0-92,11 22) compared with those in the most wellconnected group (p for trend = 0 12). Previousstudies have reported an association betweensocial isolation and smoking related cancers.13However, we found no significant associationbetween social isolation and lung cancer mor-tality (relative risk in men with fewest socialconnections= 1-48; 95% CI 0 43, 5 07; p fortrend= 0-52, based on 39 cases).

CORONARY HEART DISEASE AND STROKEINCIDENCEAltogether 104 incident cases of stroke (91cases of non-fatal stroke, and 13 cases of fatalstroke) and 403 cases of incident coronaryheart disease (275 cases ofnon-fatal myocardialinfarction, and 128 cases of fatal coronary heartdisease) occurred during the four year followup. If a participant developed a new episodeof stroke or heart attack and died within thetwo year interval between two successive followup questionnaires, then this person was clas-sified as having had an incident fatal event.A strong dose-response gradient was found

between the level of social networks and strokeincidence (p for trend, 0-008) (table 3). In-creased risks were observed for both fatal andnon-fatal stroke, although there were too fewcases of fatal stroke to carry out multivariateanalysis. In contrast to stroke, social isolationwas not associated with the incidence of eithertotal coronary heart disease or non-fatal myo-

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Table 4 Age adjusted and multivariate relative risks of total coronary heart disease, non-fatal myocardial infarction,andfatal coronary heart disease incidence according to level of social network, 1988-92

Berkman-Syme social network index

IV high III II I (low) Test for trend, p

Toul coronary heart diseaseCases 188 90 100 25Age adjusted RR 1-00 1-27 1-16 1 17

(0-99, 1-63) (0-91, 1-47) (0-77, 1-77) 0-17Multivariate RR* 1-00 1-26 1-13 1-14

(0-98, 1-62) (0-89, 1-45) (0-74, 1-73) 0-25Fatal coronary heart diseaseCases 56 25 37 10Age adjusted RR 1-00 1-16 1-41 1-52

(0-73, 1-86) (0-93, 2-14) (0-78, 2-98) 0-06Multivariate RR* 1-00 1-10 1-34 1.42

(0-68, 1-77) (0-88, 2-04) (0-72, 2-81) 0-13Non fatal myocardial infarctionCases 132 65 63 15Age adjusted RR 1-00 1-32 1-05 1 01

(0-98, 1-77) (0-78, 1-41) (0-59, 1-72) 069Multivariate RR* 1 00 1-32 1-04 1-00

(0-98, 1-79) (0-76, 1-40) (0-58, 1-71) 0-19

*Muitivariate relative risks adjusted for age (5 year age categories), time period (1988-90; 1990-92), smoking status (never,former, and current in categories of I to 14, 15 to 24, and 25 or more cigarettes per day), history of hypertension, diabetesmellitus, and hypercholesterolemia, diagnosis of angina pectoris, deciles of body mass index, parental history of myocardialinfarction before age 60 (yes/no), daily alcohol intake (0, 0-01-4-9, 5-0-14-9, 15-0-24 9, 25-0-49-9, or 50 or more g/day), andtertiles of physical activity.

Table 5 Age adjusted and multivariate relative risks of sudden cardiac death and non-sudden cardiac death, accordingto level of social network index 1988-92

Berkman-Syme social network index

IV high III II I (low) Test for trend, p

Sudden cardiac deathCases 21 10 12 2Age adjusted RR 1-00 1-24 1-22 0-82

(0-58, 2 63) (0-60, 2 49) (0-19, 3-52) 0-78Multivariate RR* 1-00 1-22 1-10 0-68

(0-57, 2-60) (0-54, 2-26) (0-16, 2-96) 0-94Non-sudden cardiac deathCases 35 15 25 8Age adjusted RR 1 00 1-12 1-52 1-94

(0-61, 2-03) (0-91, 2-54) (0-91, 4 14) 0-04Multivariate RR* 1-00 1-05 1-48 1-89

(0-57, 1-94) (0-88, 2-49) (0-87, 4-13) 0-05

cardial infarction (table 4). There was a modestincrease in risk of fatal coronary heart diseaseamong individuals with the least social con-nections (multivariate relative risk, 1-42; 95%CI 0-72, 2.81). We examined this associationin detail by separately analysing sudden cardiacdeath (45 cases) and non-sudden cardiac death(83 cases) (table 5). The rationale for per-forming this analysis was based on the theorythat social relationships enhance disease sur-vival through the provision of various types ofsupport, for example, advice to stop smoking.2'If improved survival among individuals withalready established disease is indeed the mech-anism by which social relationships affect mor-tality, we would not expect to find a protectiveeffect of social networks for sudden cardiacdeath, since by definition, death occurs withinone hour of the onset of symptoms. Consistentwith this hypothesis, we found no associationbetween level of social ties and sudden cardiacdeath (p for trend, 0-94), but a dose-responsegradient with risk of non-sudden cardiac death(p for linear trend= 0-05) (table 5).

Finally, we assessed potential effect mo-dification by smoking status and diagnosis ofhypertension. Within strata of smoking status(never, former, and current smokers), thestrongest association of social networks withtotal mortality was found among currentsmokers. The relative risk of total mortality inthe least socially connected group was 2-16

(95% CI 1-10, 4-23) among smokers, 1P73(95% CI 109, 2-76) among former smokers,and 1P10 (95% CI 0-53, 2-28) among neversmokers. The smoking X social isolation inter-action term was statistically significant(p<-001). The impact ofpoor social networkson cardiovascular mortality was stronger inmen with a diagnosis of hypertension (relativerisk, 2-62; 95% CI 1P07, 6A45) than in menwithout hypertension (relative risk, 1-57; 95%CI 0 74, 3 31). When we ran a model assessinginteraction, the hypertension X social isolationterm was statistically significant (p = 001).

DiscussionOur findings are compatible with the notionthat social networks reduce total -mortality bylowering deaths from cardiovascular diseaseand accidents/suicides. In previous studies, therelative risks of total mortality among the mostsocially isolated individuals ranged from 1-8(in the Evans County study) to 4-0 (in theGothenburg study), whereas the present studyfound a multivariate relative risk of about 1A4.The smaller magnitude of risk found in thepresent study may be partly due to the highsocioeconomic status and relative homogeneityof health professionals who continued our co-hort. High socioeconomic status - associatedwith such resources as a high level ofknowledgeabout health and disease, or access to health

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care and other material goods - may reducesome ofthe disadvantages ofsocial isolation.2223We were also able to adjust for a broader rangeof health behaviours and risk factors in thisstudy compared with previous investigations.34In terms of the magnitude of the association,an adjusted relative risk of total mortality of1-4 is comparable to the effect of cigarettesmoking on total mortality reported in somestudies.2425Four potential pathways have been proposed

through which social relationships might affecthealth.'4 Firstly they may have an affect throughthe provision of instrumental support, such asfinancial assistance or services in kind. Suchtangible assistance could contribute to therecovery and survival of patients after majorillness (such as stroke), independent of pro-fessional medical services. A second pathwayis through the provision of information andadvice that might assist individuals to seekmedical care services, or to adopt more healthpromoting behaviours, such as smoking ces-sation. The existence of such a pathway issupported by two of our findings: that sociallyisolated men were less likely to undergo medicalscreening (table 1) and, secondly, that theeffects of poor social networks were strongestamong those who smoked or had hypertension.If risky health behaviours (such as smoking)are in the pathway through which poor socialnetworks act, then control for such behavioursin multivariate analyses may constitute stat-istical over adjusment and attenuate the re-ported associations towards the null.A third postulated pathway linking social

networks to health is the provision of emotionalsupport. A recent report26 indicated that afteran acute myocardial infarction, patients wholacked emotional support were nearly threetimes as likely to die during the first 6 months(odds ratio, 2-9; 95% CI 1-2, 6-9) comparedwith patients who received emotional support.A limitation of our study is that we did notcollect data on such functional aspects of socialties. Thus, we are unable to distinguish therelative importance of different types of socialsupport (eg, emotional, instrumental, in-formational) for the maintenance of health.

It has been suggested that social networksmay directly influence host resistance and sus-ceptibility to disease through alterations inneuroendocrine and immunological controlsystems."'2728 For example, stress induces asustained increase in cortisol and insulin levels,which in turn may lead to hyperlipidaemiaand accelerate atherogenesis.29 However, fewstudies have actually demonstrated alteredneuroendocrine control under conditions ofstress.30 Importantly, our data failed to cor-roborate the hypothesis of generalised diseasesusceptibility. Increased risks were found onlyfor suicide (confirming Durkheim's original ob-servation), and cardiovascular disease mor-tality.Our analyses of coronary heart disease in-

cidence suggest that social relationships affectsurvival following the onset of coronary heartdisease, but not the incidence of new disease.A prospective study of social networks among

Japanese-Americans residing in Hawaii3' alsofound no association with incidence of non-fatal myocardial infarction. In our data, poorsocial ties were associated with an increasedrisk of fatal coronary heart disease, but only ifsudden cardiac deaths were excluded as an endpoint. The finding that social networks havelittle or no influence on the risk of suddencardiac death suggests that the protective mech-anisms associated with social ties operate afterthe onset of illness. This interpretation is alsoconsistent with several prospective studiesamong patients with established coronary heartdisease,26 32-35 in which social relationships werefound to predict survival after acute myocardialinfarction.The association of strong social ties with

reduced stroke mortality was previously de-scribed in the Alameda County study.3 To ourknowledge, no other study has reported a pro-tective effect of social networks on stroke in-cidence. Several previous studies have reporteda protective effect of social ties on cancer sur-vival,'33536 although the evidence has beeninconsistent37 38 depending on the stage ofthe cancer and the tumour site. We found noassociation of total cancer mortality with socialties, although our four year follow up periodcannot exclude an effect on long term survival.We caution that some of our disease specificfindings are based on limited duration of followup with small numbers of cases. Nonetheless,our data suggest that social networks may im-prove the outcome of established disease, espe-cially cardiovascular disease.

This study was supported by research grants HL 35464 andCA 55705 from the National Institutes of Health. We thankthe participants of the health professionals follow up study fortheir continued cooperation and participation; and Mr MarkShneyder, Ms Betsy Frost-Hawes, Ms Mitzi Wolff, Ms KerryPillsworth, Ms Karen Corsano, Mr Steve Stuart, and Ms MiraKoyfman for help with compiling the data and preparing thepaper.

1 Durkheim E. Suicide. New York: Free Press, 1951.2 House JS, Landis KR, Umberson D. Social relationships

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