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V. The Difference between Children and Adults

As a matter of every day experience, everyone knows there are striking differences between adults and adolescents. Adults are able to think before they act, to reason through situations, and to rely on logic and experience to make decisions. Adults are able to withstand pressure from others, to interpret what others do and say, and to map their own responses using judgment. These are qualities adolescents are growing towards, but have not yet fully developed.

The difference in mental and emotional functioning between adults and teenagers has prompted the emergence of an entire specialty of adolescent and developmental psychology. Professionals in this field have identified numerous traits specific to youth, including a higher propensity for risk-taking than adults and a tendency to make decisions on impulse without weighing either the short- or long-term consequences of their actions.⁵⁵

The anatomical basis in the brain for at least some of these traits can now be identified by neuroscientists. Using magnetic resonance imaging (MRI), they can produce images of the anatomy and function of the brain at different ages and while an individual performs a range of tasks.⁵⁶ Through this technique, scientists have uncovered striking differences between the brains of adolescents and those of adults. Most of the research has focused on adolescents’ frontal lobe, which is composed of different quantities and types of cell matter than the adult brain. Researchers have linked the frontal lobes to “regulating aggression, long-range planning, mental flexibility, abstract thinking, the capacity to hold in mind related pieces of information, and perhaps moral judgment.”⁵⁷ After puberty, the brain produces a dramatically larger volume of gray matter (which contains cell bodies) in the frontal lobes. Through late adolescence, the brain prunes excess neurons and their linkages in the gray matter, producing substantial declines in its volume.⁵⁸

While the brain’s composition is transformed through this pruning process, the connections between cells also change. Using MRI scanners on healthy teenagers and young adults, Elizabeth R. Sowell of the University of California, Los Angeles reported in 1999 that myelin, the fatty tissue around nerve fibers that fosters transmission of electrical signals, accumulates especially slowly during this pruning process in the frontal lobe. Myleination improves the connections between neurons in the brain. By allowing the frontal lobe’s cells to communicate more quickly and efficiently, myleination increases a person’s ability to undertake decision-making that projects into the future, and to weigh rationally the consequences of a particular course of action.⁵⁹

This late phase of myelin formation, which occurs at the end of the teenage years, provides a neural basis for concluding that teens are on average less responsible for criminal acts than adults are. According to neuropsychologist Ruben Gur of the University of Pennsylvania, there is no way to determine whether, for example, a seventeen-year-old possesses a fully mature brain. “But the biological age of maturity generally falls around age twenty-one or twenty-two,” in Gur's view.⁶⁰

Because their frontal lobes function poorly, adolescents tend to use a part of the brain called the amygdala during decision-making.⁶¹ The amygdala is responsible for impulsive and aggressive behavior, and its dominance makes adolescents “more prone to react with gut instincts.”⁶²  In adult brains, the frontal lobes offer a check on the emotions and impulses originating from the amygdale. But this check does not work to the same extent in children’s brains.⁶³

Gregory C., who was convicted of shooting a police officer who had pulled a friend and him over for speeding in a car that they had just stolen, described to Human Rights Watch acting irrationally and in response to uncontrolled impulses:

A kid just does something—whether it’s an accident or intentional. I mean personally, me, I was fifteen years old . . . I didn’t know what I was doing. I was still a kid. . . . Kids do a lot of stupid things. . . . The person I was when I was fifteen, I really didn’t have any morals, I didn’t even know who I was at that time. I hate to admit it, but I was real ignorant.⁶⁴

The MRI studies of the brain and developmental science provide a physiological explanation for features of adolescence that developmental psychologists—as well as parents and teachers—have identified for years.⁶⁵ Children are less responsible for their actions than adults because they have physiologically less developed means of controlling themselves.