On 15 November the world’s population reached 8 billion, according to the UN. Much of that growth is because we’re living longer. As a species we will continue to age, but eventually stop growing. The UN predicts that in the next century humanity will begin to go into decline. So what happens when societies get older and smaller – a problem some countries are already encountering? Ian Sample speaks to Prof Vegard Skirbekk about how humanity got here, and how we prepare for future demographic change. Help support our independent journalism at theguardian.com/sciencepod