Rewards beyond immediate physical gratification reinforce sexual intimacy among humans [1,2,3,4]. Intuition, theory and observational data suggest that these rewards include enhanced mental health, realized, for example, through greater happiness ; feelings of belonging, desirability, and self-worth [5, 6]; satisfaction with life [5, 7]; and protection against depression [5, 8]. Testing such hypotheses via experimentation has, however, proved difficult for several reasons including that ethical and practical constraints require recruitment of pre-existing couples as the unit of analysis. Couples differ on how much sexual intimacy they prefer . Randomly assigning a representative sample of couples to low vs. high intimacy will, therefore, move many away from their optimal level, thereby lowering their subjective well-being as an artifact of the design. Indeed, the only published study based on randomization found that happiness declined among those assigned to more frequent coitus . External validity of true experiments may also prove low because recruiting an unbiased sample of couples willing to comply with protocol will likely remain problematic.
Survey-based research [1, 4, 10, 11] would avoid many of the problems noted above if self-reporting of sexual behavior and well-being proved accurate and if answering questions about intimacy deterred fewer participants than does the prospect of complying with experimental protocols. Assuming those circumstances applied, a positive association between self-reported sexual activity and well-being would, however, provide only weak evidence of causation. The association could arise via reverse causation. For example, pre-existing depression could lead to reduced intimacy, and indeed the defining characteristics of depression include loss of libido .
Using self-reported appraisals of psychological states as outcomes also raises questions about the comparability and objectivity of survey-based research. Some research has used, for example, a measure of mood as an outcome [8, 9], while other studies have gauged life satisfaction  or quality of life , and still others measured domain-specific outcomes such as satisfaction with marriage or relationships . Such measures may not, moreover, characterize anything of sufficient societal importance to warrant public health, as opposed to clinical, attention.
Epidemiologists, whose methods have often proved helpful when experimentation cannot produce compelling evidence, have not attempted to test the association between sexual intimacy and mental health. They have, however, asked whether sexual intimacy predicts mortality . That literature has proved controversial for several reasons including that some scholars have made light of attempts to understand the association [15, 16].
We offer a test that avoids many of the problems noted above. We test the hypothesis that the monthly incidence of suicide, an unquestionably objective and societally important measure of mental health in a population [17, 18] decreased among Swedish men aged 50–59 after July 2013 when the patent rights to sildenafil (i.e., generic Viagra) ended. Swedish health insurance did not cover “lifestyle” drugs like Viagra, leaving consumers to pay the entire cost. After the patent rights terminated, major Swedish pharmacy chains sold 12 tablets of generic sildenafil (50 mg sildenafil) for as little as 300 SEK (about 45 USD at then-current exchange rates), about 25% of the cost for Viagra (1200 SEK, or about 180 USD) . The number of Swedish men using sildenafil averaged 62,000 per year from 2007 to 2012. That number increased to 101,000 for the years 2014–2017 .
We focused our hypothesis on men aged 50–59 for two reasons. First, the prescribing of sildenafil to treat erectile dysfunction increased most quickly in that age group in Sweden  as in other countries [22, 23]. Second, data provided us by the Swedish National Board of Health and Welfare and described below show these men have historically accounted for more suicides in Sweden than have any other 10-year age by sex group.
We argue that if sexual intimacy boosts constituents of subjective well-being and protects against depression, the high incidence of suicide among Swedish men aged 50 to 59 occurs, at least in part, due to the onset of erectile dysfunction. If correct, this argument implies that the 63% increase in men using sildenafil likely reduced the historically expected fraction of vulnerable men in the population.
Biased samples do not affect our test because our data describe the entire population of Swedish men aged 50–59. No research protocols required individuals or couples to behave contrary to their preferences for intimacy. And reverse causation does not apply—suicide among middle-aged Swedes did not cause pharmaceutical firms to lower the price of sildenafil.