During the last century-and-a-half of human history fertility rates among most urbanizing populations have reduced dramatically. There was serious concern at the beginning of the twentieth century among some medical men and some feminists that this might in large part be due to the sterilizing consequences of ‘venereal diseases’ or sexually transmitted infections (STIs). With the development of effective medical treatments, that concern declined. Since the 1940s the widely held theory of demographic transition has instead proposed that both fertility and mortality—including from STIs—have fallen in developed countries as a consequence of ‘modernizing’ economic growth delivering rising per capita incomes, improved medical technology and rising consumer aspirations. However, research on mid-twentieth-century equatorial Africa and Oceania has demonstrated that exposure to globally ‘modernizing’ forces could result in rising disease incidence, with STIs causing significant levels of infertility. Recent research has also established estimates of STI prevalence in Britain before 1914, confirming contemporary concerns that these caused increased involuntary sterility. However, this research also confirms that most of the dramatic reduction in fertility in Britain and other advanced economies was voluntary, although possibly influenced to some extent by anxieties over STIs.