“Social relationships, or the relative lack thereof, constitute a major risk factor for health - rivaling the effect of well established health risk factors such as cigarette smoking, blood pressure, blood lipids, obesity and physical activity”House, Landis, and Umberson; Science 1988 (Quoted in full article below) 
A systematic review, published in PLoS Medicine in July 2010, reviewed 148 studies involving 308,849 participants. The researchers examined studies carried out in both community populations and patient samples, and examined only the “hardest” endpoint—mortality (excluding studies in which only suicide or injury-related mortality was reported).
The researchers report that stronger social relationships were associated with a 50% increased chance of survival over the course of the studies, on average. The effect was similar for both “functional” (e.g., the receipt or perception of receipt of support within a social relationship) and “structural” measures of relationships (e.g., being married, living alone, size of social networks).
The random effects weighted average effect size was OR = 1.50 (95% CI 1.42 to 1.59), indicating a 50% increased likelihood of survival for participants with stronger social relationships. This finding remained consistent across age, sex, initial health status, cause of death, and follow-up period. Significant differences were found across the type of social measurement evaluated (p<0.001); the association was strongest for complex measures of social integration (OR = 1.91; 95% CI 1.63 to 2.23) and lowest for binary indicators of residential status (living alone versus with others) (OR = 1.19; 95% CI 0.99 to 1.44).
The authors state that the degree of mortality risk associated with lack of social relationships is similar to that which exists for more widely publicized risk factors, such as smoking.
Click here to read the editorial.
Citation: The PLoS Medicine Editors (2010) Social Relationships Are Key to Health, and to Health Policy. PLoS Med 7(8): e1000334. doi:10.1371/journal.pmed.1000334
Click here to read the full article.
Citation: (2010) Social relationships and mortality risk: A meta-analytic review. PLoS Med 7: e316. doi:10.1371/journal.pmed.1000316.