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Prior research based on Swedish data suggests that collective optimism, as measured by monthly incidence of suicides, correlates inversely with selection in utero against male twins in a population. We test this finding in the US, which reports the highest suicide rate of all high-income countries, and examine whether monthly changes in overall suicides precede changes in the ratio of male twin to male singleton live births. Consistent with prior work, we also examine as a key independent variable, suicides among women aged 15−49 years. We retrieved monthly data on suicides and the ratio of male twin to singleton live births from CDC WONDER, 2003 to 2019, and applied Box-Jenkins iterative time-series routines to detect and remove autocorrelation from both series. Results indicate that a 1% increase in monthly change in overall suicides precedes a 0.005 unit decline in male twin live births ratio 6 months later (coefficient = −.005, p value =.004). Results remain robust to use of suicides among reproductive-aged women as the independent variable (coefficient = −.0012, p value =.014). Our study lends external validity to prior research and supports the notion that a decline in collective optimism corresponds with greater selection in utero. [ABSTRACT FROM AUTHOR]