Birth rates are collapsing across the developed world, and the situation is far worse than most people realize — bad news for any economy that depends on a growing population.

• Japan's fertility rate is 0.7, which means the population halves every generation.
• Zoo animals stop breeding when their enclosure is unfit, and something similar may be happening to people.
• Modern life is the "enclosure": something about how people now live is so stressful that a whole generation is opting out of having children.
• Few governments or markets have priced in what a halving population means for workers, pensions, and growth.

Outlook: Without a real change in how people live, birth rates will keep falling and shrinking populations will become one of the biggest long-term economic problems.