Populations function as complex systems, and disease rates might result from the characteristics of individuals in the population, the interactions and interdependences between individuals, the effects of population-level factors on individual-level health outcomes, and the interplay between individual-level and population-level factors.1 This view of population health provides a best fit to understanding the importance of social determinants of disease and health within populations.4 Recent analyses in the USA demonstrate that reported sexual behaviour in terms of number of partners, practices, condom use, and other individual-level factors does not solely account for the observed racial and ethnic disparities in rates of HIV or other STIs.5 In most analyses, African American women have among the highest HIV and STI prevalence but do not have the highest levels of risk behaviours. Furthermore, the sex ratio of men to women is much lower among African Americans than all other ethnic groups as a result of high mortality rates among black men from disease and violence, and high rates of incarceration.1*17 These two social forces, poverty and the low sex ratio, are probably among the biggest determinants of sexual network patterns among black people in the USA.* Poverty and low sex ratios are associated with low marriage rates, and married people have lower rates of concurrent partnerships than unmarried people.18 Moreover, the shortage of black men promotes partner concurrency as well as partnering between women with low-risk behaviours and men with high-risk behaviours.