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. They need care and feeding. They can’t be trusted to cross busy roads, sweep chimneys or hunt mammoths for ages. They’re sitting ducks for sabre-toothed tigers. Considerable investment of time and energy in growing them to physical maturity (not to mention the opportunity cost of not being able to procreate for a bit) is made by the parents after birth.
Some animals spread their bets – giving birth to very many infants in the hopes that enough survive to breed. Some, like us, tend to bet the farm on a relatively small number. Seasonal scarcity of food, the presence of predators and all sorts of other factors affect when and how animals mate and give birth, how long they care for their children, and when those children start making children of their own. With a wide range of reference, Hassett sets out to put our human investment strategies in their evolutionary context.