Background: Suicide is a major public health problem. Current thinking about suicide emphasizes the study of psychiatric, psychological, or biological determinants. Previous work in this area has largely relied on surrogate outcomes or samples enriched for psychiatric morbidity.

Objective: To evaluate the relationship between social integration and suicide mortality.

Design: Prospective cohort study initiated in 1988.

Setting: United States.

Participants: 34,901 men aged 40 to 75 years.

Measurements: Social integration was measured with a 7-item index that included marital status, social network size, frequency of contact, religious participation, and participation in other social groups. Vital status of study participants was ascertained through 1 February 2012. The primary outcome of interest was suicide mortality, defined as deaths classified with codes E950 to E959 from the International Classification of Diseases, Ninth Revision.

Results: Over 708,945 person-years of follow-up, there were 147 suicides. The incidence of suicide decreased with increasing social integration. In a multivariable Cox proportional hazards regression model, the relative hazard of suicide was lowest among participants in the highest (adjusted hazard ratio [AHR], 0.41 [95% CI, 0.24 to 0.69]) and second-highest (AHR, 0.52 [CI, 0.30 to 0.91]) categories of social integration. Three components (marital status, social network size, and religious service attendance) showed the strongest protective associations. Social integration was also inversely associated with all-cause and cardiovascular-related mortality, but accounting for competing causes of death did not substantively alter the findings.

Limitations: The study lacked information on participants' mental well-being. Some suicides could have been misclassified as accidental deaths.

Conclusion: Men who were socially well-integrated had a more than 2-fold reduced risk for suicide over 24 years of follow-up.

Primary funding source: National Institutes of Health and Robert Wood Johnson Foundation.