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Sex Ratios, Sexual Infidelity, and Sexual Diseases: Evidence from the United Kingdom

We extend a theory of fidelity in a two-sided economy, and empirically discriminate between different rationales of sexual network formation by testing their implications for how sex ratios affect sexual activity, relationship stability, and the spread of sexually transmitted diseases in men versus women. We use a unique individual-level dataset in combination with census data from England and Wales, a setting where adult women outnumber adult men. Exploiting variation in cohort/ethnicity/region-specific sex ratios as a quasi-natural experiment, we find that a decrease in sex ratio imbalance decreases sexual infidelity and the number of serial partners, and increases the likelihood of safe sex. This in turn reduces the likelihood of acquiring a range of sexually transmitted infections and diseases, including chlamydia, gonorrhoea, genital warts, and herpes. Consistent with the rationale underlying the formation of egalitarian (in)fidelity networks, the effects of the sex ratio on sexual activity are larger for men compared to women, while its effects on sexual diseases are larger for women compared to men. The causality of these effects is established using classical and recent instrumental variables approaches and various robustness checks. For falsification, we show that sex ratios have no impact on several ”atheoretical” health conditions, such as Parkinson’s disease, chronic lung disease, heart attack, stroke, and diabetes, which do not arise from sexual interactions.