The prevalence of HIV infection in Africa is substantially higher among young women than it is among young men. Biological explanations of this difference have been presented but there has been little exploration of social factors. In this paper we use data from Carletonville, South Africa to explore various social explanations for greater female infection rates. This paper reports on data from a random sample of 507 people between 13 and 24 years old. Subjects were tested for HIV, as well as other sexually transmitted infections (STIs), and answered a behavioural questionnaire. The age-prevalence of HIV infection differs between men and women with considerably higher rates of increase with age among young women. The age of sexual debut did not differ significantly between men and women (15.9 and 16.3 years, respectively) and below the age of 20 years there was no difference in the number or distribution of the number of sexual partners reported by men or women. The risk of infection per partnership was substantially higher among women than among men. Women have sexual partners who are, on average, about five years older than they are with some variation with age. Scaling the age-prevalence curve for men by the age of their sexual partners gives a curve whose shape is indistinguishable from that for women but is about 30% lower for men than for women. In terms of social explanations for HIV rates among women, the data indicates that this difference can be explained by the relative age of sexual partners, but not by other factors explored. In addressing the epidemic among young women it will be essential to deal with the social factors that lead young women to select their partners from older-age cohorts and that shape their sexual networking patterns.