Sex Ratios, Sexual Network Formation, and Sexual Diseases : The Role of Gender

How does the sex ratio affect sexual network formation, relationship stability, and the spread of sexually transmitted diseases, and how do these effects differ by gender? We address these questions by developing a dynamic theory of sexual network formation in a two-sided economy, where agents enjoy having many sexual partners but do not want their partners to have others. We fully characterize steady state sexual networks. This characterization implies that increasing the male-to-female ratio decreases sexual infidelity and the spread of sexually transmitted infections (STIs). The effect on sexual infidelity is larger for men compared to women, whereas the effect on STIs is larger for women compared to men. We test these predictions of the model using a unique individual-level dataset, in combination with census data from England and Wales. Exploiting variation in cohort/ethnicity/region-specific sex ratios as a quasi-natural experiment, we find that an increase in the sex ratio decreases sexual infidelity and the number of serial partners, and increases the likelihood of safe sex. It also reduces the likelihood of acquiring a range of STIs including chlamydia, gonorrhoea, genital warts, and herpes. In addition, the differential effects of the sex ratio on men and women validate the model's prediction. These findings are robust to using classical and recent instrumental variables approaches and various robustness checks. We also conduct a placebo test, showing that the sex ratio has 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