As adults, however, we have a tendency to err too far towards exploitation – we become content to fall back on the stock of knowledge and mental habits we built up when we were young, rather than adding to or revising it. We get lazy.

There are neurological reasons for this. Though the adult brain is more flexible and volatile than was once thought, it is still less malleable than the teenage brain. Reconnecting neurons and rewiring synapses is hard work.5 But in the twenty-first century, you can’t afford stability. If you try to hold on to some stable identity, job, or