Sex acts, sex partners, and sex budgets: implications for risk factor analysis and estimation of HIV transmission probabilities.

Many epidemiological studies have identified the number of sex partners as a risk factor for the acquisition of HIV, but few studies have identified the number of sex acts as a risk factor. The seeming lack of importance of the number of sex acts as a risk factor has yet to be explained. In this report we conduct an exploratory data analysis to evaluate the relationship between the number of sex acts and the number of sex partners for heterosexuals. Our results indicate that it may be most appropriate to view sexual activity within a sex budget and resource allocation framework. We use the results (a) to suggest an explanation for why the results from some of the risk factor analysis studies have identified a per partnership but not a per act risk, and (b) to assess the implications of the relationship for the estimation of heterosexual transmission probabilities for HIV.