Our long, vulnerable childhoods may be the key to our success

The central question in Brenna Hassett’s book, put simply, is: why are our children so very useless for so very long? Or: ‘What is the possible adaptive value of teenagers?’ If we consider maturity, or adulthood, to be the point at which an animal can play its own role in the evolutionary process – i.e. have its own babies – why is it that we have evolved to mature so slowly; and, even when mature, to delay having children until many years after we’re first physically capable of doing so?

The framework in which Hassett sets out to answer this is one to do with investment and return on investment. An animal invests energy in growing its young. Sometimes that energy is front-loaded in gestation: infant giraffes come out more or less fully baked, or precocial, and are making their knock-kneed way across the savannah soon after birth. In other species, infants are altricial: they come out like baby rats or baby humans, helpless.