American Scientist

Why Youth Is Not Wasted on the Young: Immaturity in Human Development. David F. Bjorklund. xii + 276 pp. Blackwell Publishing, 2007. $24.95.

Why do children take so long to grow up? From an evolutionary perspective, the fact that humans reach maturity so slowly, relative to other species, is a puzzle, because the leisurely pace of their development clearly has costs: It requires greater parental investment and increases the risk that offspring will die before reproducing.

One possible explanation for our slow rate of maturation is that it is an adaptation—that is, natural selection may have favored a long childhood because it had benefits that outweighed its costs. However, most scientists who have examined this issue have assumed that immaturity has no inherent advantages and that our extended period of development must therefore be a by-product of selection for some other characteristic.

The most popular candidate has been intelligence. A big and complex brain takes a lot of time to develop, and in humans much of that development must occur after birth, because bipedalism limits birth-canal width, which has in turn constrained the head size of newborns. More specifically, social intelligence has often been postulated as the driving factor. In this view, as humans achieved ecological dominance, they became one another's principal competition for resources. Consequently, the ability to manage social relations and alliances was selected for, in what evolutionary biologist Richard Alexander has characterized as a cognitive "arms race" within the species. The result is that we are much smarter than we would need to be simply to succeed at hunting and gathering, and we are thus capable of creating complex cultures.

David F. Bjorklund, an influential leader in the emerging area of evolutionary developmental psychology, does not dispute the foregoing account in his new book, Why Youth Is Not Wasted on the Young. But he does question the assumption that extended immaturity has no benefits. He suggests that a long period of development, although it may have arisen as a by-product of other factors, does confer advantages—behavioral flexibility, for example—that have contributed to our success as a species. In other words, he believes that childhood is an evolutionary spandrel (although he doesn't use the term)—a side effect, which in this instance turns out to be adaptive itself.

Children's thinking differs from that of adults, and people tend to view those differences as deficits that need to be overcome—the sooner the better. Bjorklund argues, though, that some aspects of children's immature cognition are actually adaptive, both in preparing them for adulthood and in allowing them to flourish in childhood. He gives several examples: Children typically overestimate their own abilities, which may maintain their motivation in the face of failure and lead to eventual success. Their limited information-processing capacity may help them learn language, because it forces them to focus on constituent components and build upward from there, whereas adult language learners skip straight to semantics, often failing to master underlying grammatical structures. And play in childhood may promote later social competence, as neuroscientist Sergio Pellis has demonstrated in rats.

Bjorklund stops short, however, of claiming that a long childhood is a genuine adaptation. He acknowledges that maturity is still the goal of development and that adults shouldn't try to extend childhood artificially "by 'babying' children." He simply argues that slow development has some benefits that may, in concert with other selection pressures, help explain why it evolved. He also maintains that childhood should be appreciated and not rushed.

Although Bjorklund contends that immaturity has benefits, he doesn't come up with very many. He is still using mostly the same examples that he and Brandi Green included in "The Adaptive Nature of Cognitive Immaturity," their classic 1992 article in American Psychologist.

In several places in his new book, Bjorklund appears to be on the verge of endorsing the fallacious argument that big brains and slow development evolved to allow individuals to learn about the complexity and variability of human cultures. (In fact, human cultural complexity and variability could not exist without big brains and slow development and thus cannot be the cause of them, as a cause must precede its effects. Culture cannot arise independently, without individuals capable of creating it.) Fortunately, Bjorklund catches himself before falling into this trap. He asserts instead that perhaps these characteristics coevolved:

There is no simple cause and effect here; the relation among these three factors [social complexity, big brains and slow development] is synergistic, with changes in one factor being both a cause and a consequence of changes in related factors. But social complexity was a required ingredient in human cognitive evolution. It exerted selection pressure for a bigger brain and a prolonged childhood, which in turn permitted increased levels of social complexity to be attained.

Bjorklund finds implications that will interest parents and educators. Parents often want their children to be the first among their peers to reach every developmental milestone, but Bjorklund points out that earlier is not always better and may sometimes be worse.

For example, abnormally early visual experience in birds disrupts development of the auditory system. Also, in a classic study published in American Scientist in 1959, "The Development of Learning in the Rhesus Monkey," psychologist Harry Harlow found that the ability of rhesus monkeys to discriminate objects on various dimensions such as shape was impaired by starting the training too early in life—the monkeys who started training at older ages reached higher peak levels of performance. In a 1977 study by developmental psychologist Hanus Papousek, human infants who started learning to turn their heads to specific sounds at 31 days of age mastered the task, on average, at 71 days of age, whereas infants who started learning to do so at birth did not master the task, on average, until the age of 128 days.

Bjorklund's message is that human development takes as long as it does for good reasons and that experiences should be introduced only when children are cognitively ready for them. Early education should foster a love of learning, which will pay dividends in the long run, rather than a fear of falling behind, which increases stress and decreases motivation. He acknowledges that schooling is necessary for success in the modern world and that direct instruction is sometimes useful. But as much as possible, he believes, we should let children enjoy childhood. We should even seek to maintain some "immature" qualities, such as curiosity and playfulness, into adulthood. As Aldous Huxley observed, "The secret of genius is to carry the spirit of the child into old age, which means never losing your enthusiasm."

Bjorklund is well qualified to write authoritatively on this topic, as evidenced by his seminal 2002 book The Origins of Human Nature: Evolutionary Developmental Psychology (which he coauthored with Anthony Pellegrini) and the essential 2005 collection Origins of the Social Mind: Evolutionary Psychology and Child Development (which he coedited with Bruce Ellis). Fortunately, Bjorklund does not view everything through the lens of his particular research area. He is a successful textbook author, with a breadth of knowledge that allows him to draw examples from across the field of developmental psychology.

Why Youth Is Not Wasted on the Young is that rare sort of science book that will be interesting to researchers as well as to laypeople and readers from other fields. Bjorklund provides enough background for anyone to understand his arguments. However, focus on the overall argument is lost in occasional passages that read like a textbook, listing concepts and terms. Fortunately, the book is enlivened by amusing anecdotes, such as one in which Bjorklund recalls how he, as a first-grader, discovered in front of a room full of his classmates that he had overestimated his ability to tap dance. The content is accessible enough for use with undergraduates, yet sufficiently meaty for a graduate course. And throughout, Bjorklund is a beautifully smooth writer.