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.
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