Evolutionary researchers have long puzzled over suicidal behavior. In this
paper, we propose that when people are unable to achieve fitness-enhancing
outcomes, including attracting a mate given unfavorable sex ratios, suicide
rates increase. We test whether suicide rates are associated with
county-level sex ratios utilizing data from the CDC’s Underlying Cause of
Death database from 1999-2018, controlling for a variety of factors known
to associate with suicide risk, including ethnic background, income,
unemployment, veteran status, population density, and population growth or
decline. We find that sex ratio is associated with suicide risk, where a
greater proportion of males in a county (age 35-74) is associated with an
increased risk of suicide for these males. Mediation analyses show that
this effect is mediated by male marriage rates. Counter to predictions,
male-biased sex ratios also tend to be associated with *increased* female
suicide rates between ages 35 and 74, and this effect is mediated by
unmarried sex ratio. We discuss possible reasons for this counter-intuitive
finding. Overall, these results suggest that male-biased sex ratios
influence men’s ability to marry and ultimately suicide risk, but
limitations exist.