Consider three women, all known to me in various ways, all born in the same year, now in their mid-forties – a widow, a wife and a singleton. Samira, living in Niger, a country with the highest child-bearing rate in the world, was married at thirteen to a 52-year-old friend of her father. Before she was widowed at twenty-eight, she had had fifteen pregnancies and nine live births. She is now completely dependent on her husband’s family to provide her and her children with housing, food and clothing. Varya, whom I met several times on visits to Asia, was born in rural Malaysia. She travelled to Singapore at the age of sixteen to find work as a cleaner in a large American-owned hotel, living in a hostel for fifteen years. Under family pressure, she returned home, marrying a local boy and bearing a son in her early thirties. Varya and her husband have decided to stop at two children. Varya does not want to stop working in Singapore – though it does involve twelve-hour shifts and a daily four-hour round bus trip – and she would like her children to have the ‘best of everything’: the possessions and opportunities that she was denied. Last, Lisa, an Italian whom I met in the UK, got a degree in Milan, lives in London, works in publishing and is child-free by choice. She knows that her mother is disappointed that neither she nor her brother has had children, and that she worries about who will care for them in their old age. These are three 21st-century women whose lives have been framed by changing demographic and societal circumstances. Demographic factors shape whether our lives are long and healthy or short and threatened by disease and ill health, whether we grow up in a large birth cohort needing to compete for work, housing and partners, whether we live in large family groups or alone.

The world is undergoing unprecedented population change. Some two hundred years ago, Europe began the transition from majority young to majority older populations. Asia and Latin America started this process last century and will complete it in around a hundred years. And now Africa is beginning the transition.