Approximately 40% of women in sub-Saharan Africa marry before their eighteenth birthday1. Within the international development sector, this phenomenon is referred to as 'child marriage', widely equated to forced marriage, and recognized as damaging to multiple dimensions of female well-being1,2. An escalating global campaign to end early marriage typically assumes that its high prevalence is driven by a conflict of interests between parents and daughters, with parents coercing daughters to marry early for the parents' economic benefit3. However, a parent-offspring conflict model of early marriage has not been explicitly tested. Here we present a study of marriage transitions in rural Tanzania, where marriage before or just after 18 years of age is normative. Consistent with parental coercion, we find that bridewealth transfers are highest for younger brides. However, autonomy in partner choice is very common at all ages, relationships between age at marriage and female well-being are largely equivocal, and women who marry early achieve relatively higher reproductive success. We conclude that, in contexts in which adolescents have autonomy in marriage choices and in which marriage promotes economic and social security, early marriage may be better understood as serving the strategic interests of both parents and daughters.