Over the past 150 years, the age of puberty onset has fallen substantially across many developed countries. Although trends are apparent in both sexes, 1 the evidence in females (where biological markers are clearer) suggests that, for instance, in northern Europe the age at menarche (first menstruation) fell during the 1800s, then further reduced by up to 3 years over the last century (fig 1). Factors contributing to this fall include a combination of public health successes and changes in social structures. Thus, successes such as improved childhood nutrition and health status through reduction in childhood infections have been major factors accelerating the onset of puberty. 7 Socially, however, stress is also a puberty accelerator, with familial disruption, including father absenteeism, being one of the most effective stressors, and levels of divorce as well as singleparent families have rapidly escalated in many countries (eg, England, 2005). 8 9 The sum effect of these changes has been relatively recent reductions in the age of puberty onset. 7 However, these have not been matched by efforts to socially develop young people at an equally accelerated rate, leaving an increasing gap between physical puberty and social puberty (the age at which people are mentally, educationally and legally equipped to function as adults in modern societies). Here, we propose that this disparity may underpin many of the major public health challenges associated with young people today. Puberty is a physical preparation for adulthood that, along with bodily changes, promotes interest in sexual activity, 10 increases aggression in adolescents, 11 encourages curiosity and can escalate risk-taking behaviours as people compete for social status and attempt to conform to perceived peer norms. 12 Increasingly, however, as social puberty lags behind physical puberty, the results can be ill-informed healthdamaging behaviour. Thus, early sexual activity is associated with unprotected sex and, consequently, sexually transmitted infections and teenage pregnancies. 13 Moreover, adolescent stresses resulting from mismatches in physical and social development may also promote substance use (including alcohol, tobacco and drug use) as ways of both self-medicating and trying to conform to peer pressures. Further, a lack of knowledge of how to adapt to changes in physical, mental and social status may lead to self-harm, violence and bullying. 14 Attributing all recent changes in sexual health, substance use and violence to earlier puberty (on a population basis) is oversimplistic, but disregarding the role of earlier physical maturation in these major health trends is equally inappropriate. Dissociation of physical and social puberty is also likely to affect certain communities disproportionately (eg, the most deprived). For example, early menarche has been linked with a combination of poor prenatal nutrition and excess nutrition in childhood. 7 Although studied mainly in immigrant populations, such combinations are also more likely to occur endemically in deprived areas. Equally, other factors capable of accelerating puberty, such as single-parent families, are also associated with deprived populations (eg, in the UK). 9 Surprisingly, the contribution that such inequalities make to current variations in the onset of puberty, and how they then affect patterns of sexual health, substance use and violence, is largely unstudied. The continued reduction in the age of onset of puberty should not be treated as a biological anomaly. It is likely that some 20 000 years ago, humans had already evolved to experience menarche at around 12 years and at present many countries are moving back to this …