Arguably, this collapse in fertility in high-income (and many other) countries, together with the burgeoning population of Africa, the poorest continent, is the single most important thing now happening to humanity. There are many explanations for the long-term decline in fertility, notably, falling infant mortality, mass urbanisation, the prolonged and costly dependence of children, the “rat race” of producing economically successful children and, not least, the ability to rely on other forms of security in old age, notably pensions and care homes. Also crucial, however, especially for the most recent declines to ultra-low fertility rates, is the conflict between improved opportunities for women and the disproportionate share of the economic costs (and physical risks) of children they continue to bear.