People have a lot of kids because they have, subliminally at least, an economic value in poor societies. This didn’t lead to rapid population growth when infant mortality was incredibly high, but it does now that, even in very poor countries, life expectancy has grown and infant mortality collapsed (a good thing). People will use contraception when they have no economic incentive to have lots of kids.
On a totally different subject, I didn’t realise that this is the first time ever in the UK that members of a political party will, in choosing their leader, will appoint the Prime Minister. Never happened before.