Sex Ratios, Sexual Network Formation, and Sexual Diseases

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 a larger 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 with women, whereas the effect on STIs is larger for women compared with men. We test these predictions of the model using a unique individual-level data set, 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 a larger sex ratio decreases sexual infidelity and the number of serial partners, increases the likelihood of safe sex, and reduces the likelihood of acquiring STIs. In line with the model, these effects vary by gender.