After nearly 30 years of HIV epidemics around the world, understanding of the diversity and the long-term dynamics of sexual behaviors remains limited. The debate about sexual behavioral change and HIV is often narrowed into considerations of A, B, C (abstinence, be faithful, and condom use) or concurrent versus serial sexual partnerships, and in too many epidemiological studies, sexual behaviors are extracted from their cultural context and reduced to single variables [1–3]. Such reductionist approaches give rise to conventional recommendations on HIV prevention ‘messages’ addressed to individuals, with the naive belief that they will lead to important behavioral changes at the population level.
This is why the paper of Bajos et al. in this issue  describing and analyzing changes in sexual behaviors over recent decades in France is particularly refreshing: it reminds us how much the evolution of sexual behavior, far from being a health-related behavior, is determined by changes in nuptiality and broader social structural factors.
In their study in France, Bajos et al. show that, over 70 years, the lifetime number of partners reported by men has not changed and their age at first sex has declined only slightly. Most of the changes concerned women: their median age at first sex dropped from 22 to 17.6 years and their reported number of sexual partners more than doubled. Decade after decade, women reported a much more varied sexual repertoire and enjoyed sex much more.
Similar surprising stability of men's sexual behavior but profound changes in women's sexuality over past decades have been described in other high-income countries [5,6].
The increasing disjunction or disconnect for women of the nexus ‘marriage, sex and reproduction’ is operating in nearly all high-income countries, but with great variability and at different speeds. These changes in women's sexual behavior accompany an evolution known to demographers as the ‘Second Demographic Transition’. The term was introduced by Lesthaeghe and van de Kaa  to explain demographic trends in western countries from the 1960s to the present, with an emphasis on gender relations. The concept encompasses decreased fertility and rises in modern contraceptive practice by women, increases in age at marriage and nonmarriage, in divorces and separations, and in the number of informal partnerships of varying duration.
All sexual behavior surveys in high-income countries portray a similar pattern with some variations: most of changes are related to women and all changes carry increased risk of sexually transmitted infections (STIs): the steep rise in premarital sex, earlier age at first sex, more casual partners, and greater diversity of sexual practices.
To explain the gender convergence in France, Bajos et al. identified ‘the increased social autonomy of women in other spheres of life; their increasing participation in the labor force and the postponement of childbearing’. One can add the massive increase in the education of women.
The degree to which most changes in sexual behavior will stem from women in other regions remains unknown. In recent decades, female age at marriage has risen most rapidly in Asia and the Arab states, but premarital sex among women is not common . In Latin America, sexual activity before marriage among single women increased in the past decade, as did the use of condoms and other contraceptives. Pregnancy rates for all single women aged 15–24 years rose, because the decline in virginity was greater than the rise in contraceptive protection .
In six countries in sub-Saharan Africa, a recent study of age at first sex, age at first marriage, and time spent single showed few changes over time, except for an increase in premarital sex for women . In parts of sub-Saharan Africa where patriarchal or gender-stratified cultures are not prevalent, women's autonomy has often resulted in convergence between men and women's risk behavior .
In future decades, globalization and urbanization, together with progress in education, are likely to continue to provoke improvements in women's status that will be reflected in changes in sexual behavior and all indications point to more risk behavior for HIV and other STIs.
Bajos et al. make the point that public health interventions are unlikely to succeed if they go against the force of secular trends. In response to the first HIV prevention campaigns in the mid-1980s, spectacular increases in condom use occurred and have been sustained over time in France, as in other high-income countries. In recent years, both in Africa and Latin America, condom use during risky sex outside marriage has increased substantially to reach typically about 30% in the most HIV-affected countries but not within cohabiting relationships [9,12,13]. The use of this male-initiated barrier method may become widespread only in countries where women's status and gender relations have changed radically.
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