Behave: The Biology of Humans at Our Best and Worst

This chapter focuses on neuroscientific explanations for adolescent behavior, relying on a crucial fact: “the frontal cortex [does not go] online until our mid-twenties” (154).

Since the FC is all about rationality, logic, and making decisions, its developmental delay helps to explain why we are at our most risk-taking, novelty-seeking, and social in adolescence. As such, this event of developmental psychology is another example of how understanding brain science aids us in deciphering seemingly unscientific aspects of our lives, like why teenagers behave in the strange ways they do.

Surprisingly, considering it is offline, the FC actually has more neurons in adolescence than adulthood. During late adolescence, these neurons are pruned to make not “more brain” but “more efficient brain” (157). Therefore, as adolescence progresses, connections become quicker, and different cortical regions become more densely connected, but early in adolescence these connections are less strong. The dual pruning of the frontal cortex and integration of cortical regions throughout adolescence is shown in cognitive changes in adolescence. During adolescence, working memory, flexible rule use, and task-shifting improves accompanied by increased activity in the frontal region during tasks. Though Sapolsky does not directly state it here, this cognitive pruning is also an indicator of why the interests and activities we take up in adolescence tend to shape the interests and skills we carry for the rest of our lives: our brains are designed this way.

Older teenagers experience emotion more intensely than children or adults due to lesser vmPFC suppression of amygdaloid response (this region isn’t fully online yet). Less prefrontal activation in risk assessment also makes adolescents more prone to risk-taking, even when advised against it. Adolescents are also more novelty seeking, a phenomenon found across many mammals but particularly strong in primates, as this is a period where adolescents seek out new social groups in this species. This is an example of a trait in human culture (increased sociality in adolescents) derived from our ape ancestors, where it was all about reproduction. Dopamine signaling and dopamine density increase steadily in adolescence, causing higher highs and lower lows when anticipating reward, accounting for much of adolescent emotional instability.

More highly social than adults or children, adolescents are also more subject to peer pressure. Peer encouragement in risk-taking further hinders the already underactive vmPFC. In experiments on social snubbing, vlPFC activation (the region of the PFC responsible for rationalizing feelings of anger, pain and disgust in such situations and returning to emotional stability) is less active. In other words, teens need to fit in more because rejection hurts more.

Adolescents are better at perspective-taking (seeing things someone else’s way) than children, but not as a good as adults. They correctly view unintentional harm and damage as less severe than intentional harm and damage, and over time become more oriented to solving problems than assigning blame. Adolescents are also highly adept at empathy. This combined with more general high emotionality and egoism explains their high engagement in social causes. However, their tendency for hyperarousal and vmPFC activation often inhibits effective action-taking in this regard. Even these complex emotional and social aspects of teenage life can be explained via the slow development of a single brain region.

This leads to the core question: why does the frontal cortex mature so late? Remember the primate urge for social interaction in adolescence: the risk taking of adolescence is advantageous to their chances for reproduction (as long as you don’t risk too much and end up dead). Furthermore, once we achieve social success, the navigation of this success to maintain it is the domain of the FC, which to execute this role well must be well-shaped by years of learning and pruning. In other words, the FC comes fully online at the moment when its need to exist and need to be fine-tuned by neural pruning are at an ideal balance.