People around the world are living longer and, on the whole, having fewer children. What does this mean for future populations? Sarah Harper CBE, Professor in Gerontology at the University of Oxford, tells presenter Jim Al-Khalili how it could affect pensions, why it might mean we work for longer, and discusses the ways modern life is changing global attitudes to when we have children, and whether we have them at all. Fertility and ageing have been Sarah's life's work and she tells her story of giving up a career in the media to carry out in-depth research, and going on to study population change in the UK and China, setting up the Oxford Institute of Population Ageing and later becoming a Scientific Advisor to UK Government.

Presenter: Jim Al-Khalili