It's hard to think about populations without bringing up Thomas Malthus, the 18th century political economist who predicted that humans would bring about their own collapse by growing faster than their food supply.
Malthus was a controversial figure — his theories and figures led to Charles Darwin and Alfred Wallace's work on evolution and natural selection, but they also led to some horrific policies on the part of politicians, who used them to enact population control measures like forced sterilizations and eugenics.
Many others have followed in Malthus's gloomy footsteps, predicting catastrophe brought on by overpopulation, notably Paul Ehrlich, who had a bestseller in 1968 with The Population Bomb.
Nevertheless, humanity has persisted, at least so far. But research published in The Lancet suggests that humanity isn't out of the woods just yet, but it's not food supplies that have us in trouble.