Objective To examine the frequency of three sexual behaviours from the most active to the least active members of the population in various subpopulations using measures of inequality.

Methods Data from a US national probability sample of the population aged 15–44 years (National Survey of Family Growth) were used. Gini coefficients and Lorenz curves were calculated in order to examine the concentration of three sexual behaviours: vaginal sex acts (past 4 weeks) and number of opposite-sex partners (past 12 months; lifetime). Analyses were conducted separately for men and women and subpopulations of interest (by age, race/ethnicity, educational level and poverty level).

Results The sexual behaviours examined were concentrated within the most active members of the population. This concentration was most pronounced for vaginal sex acts in the past 4 weeks and lifetime opposite-sex partners, with the top 5% of each population accounting for more of the sexual behaviour than the bottom 50% of the population. Sexual behaviours were most concentrated among adolescents, the least educated and the most impoverished. Some subpopulations had similar mean or median numbers of sex acts (or sex partners), but had different degrees of concentration of these behaviours. Finally, the most impoverished men and women had the highest concentration levels for two of the three sexual behaviours (vaginal sex acts, opposite-sex partners in past 12 months).

Conclusion Given that sexual behaviours tended to be highly concentrated in subpopulations that are often at the highest risk of sexually transmitted infections, targeted interventions may be the most efficient method to reduce risk in these groups while minimising potential unintended consequences.