I'm very aware that this programme may be regarded as bleak or depressing - an increasing population with an ever decreasing supply of resources – but humans have capabilities that animals don't: to think rationally; to study; to plan ahead. The number of people on the planet depends on the personal decisions we each make regarding the number of children we have even setting aside the moral responsibility we have to protect other species, if we continue to damage our ecosystem, we damage ourselves. Its clear that we'll have to change the way that we live and use our resources. We're at a crossroads where we can chose to cooperate or carry on regardless. Can our intelligence save us? I hope so.

How Many People Can Live on Planet Earth? (BBC Horizon, 2009)