In the 1960s there was the 'Population Explosion'. In the 1970s the notion became unfashionable, but it didn't go away. The number of Kenyans, for instance, is likely to double in the next two decades. And it is estimated that India will be home to 1,500 million by the next century.
It is clear that this multiplication cannot continue indefinitely - eventually people would weigh more than the rest of the universe. So what will stop it?
Colin Tudge explores the influences that will eventually stabilise the world's population, and considers what we might lose by allowing the crowd to become too large.
Producer GEOFF DEEHAN