The adolescent brain is not a broken or defective adult brain. It has been exquisitely forged by the forces of evolution to have different features compared to children or adults, but these differences have served our species well. The three most robust adolescent behavioral changes are (1) increased risk taking; (2) increased sensation seeking; and (3) a move away from parent toward greater peer affiliation. That these changes occur not only in humans, but in all social mammals, suggests a deeply rooted biology, which fosters independent functioning and separation from the natal family.
Another highly adaptive feature of the adolescent brain is its ability to change in response to the demands of the environment. This changeability is often referred to as “plasticity” and it is a defining feature of the human brain. The fossil record shows a tripling of braincase volume between Homohabilis and Homosapiens, followed by a slight decrease over the course of 30,000 years of human civilization. It is no coincidence that our brains are also adaptable over the lifecourse. We descend from a long line of ancestors who were able to choose the former of the “adapt or die” proposition.
Brain plasticity is a lifelong process but tends to be most robust earliest in development. Compared to other species humans have a very protracted period when we are dependent upon our parents or other adults for survival. A benefit of this protracted period of protection is that it allows our brains to stay flexible to changing demands, even more so than our close genetic kin, the Neanderthals, whose tool use changed remarkably little in over 1 million years [2
]. They were well suited to deal with a stable, albeit harsh, environment at the time but less facile at adapting to changing demands.
Humans, on the other hand, are remarkably adaptable. We can survive everywhere from the frigid North and South poles to the balmy islands on the equator. With technologies developed by our brains we can even live in vessels orbiting our planet. Survival skills in cold climates may entail learning how to find shelter and obtaining nutrients from hunting. In tropical climates it may be more a matter of avoiding certain predators or identifying which fruits are edible and which are toxic. The changes in demands across time are as striking as the changes across geography. Ten thousand years ago, a blink of an eye in evolutionary terms, we spent much of our time securing food and shelter. Modern humans now spend relatively little time and energy obtaining calories (a factor that may, through epigenetic or other factors, be related to earlier puberty and greater height/weight). Instead many of us spend the majority of our waking hours dealing with words or symbols – a particularly noteworthy departure given that reading, which is approximately 5,000 years old, did not even exist for most of human history. Having a highly plastic brain is particularly useful during the second decade when the evolutionary demands of adolescence – being able to survive independently and reproduce - rely critically on the ability to adapt.
Insight into the neurobiology of the developing brain has been greatly enhanced by the advent of magnetic resonance imaging (MRI), which allows exquisitely accurate pictures of brain anatomy and physiology without the use of ionizing radiation (see [3
] for review).
After puberty the brain does not mature by growing larger; it matures by growing more specialized. Gray matter volumes during the first three decades of life follow an inverted “U” shaped developmental trajectory with peak size occurring at different ages in different regions. Total cortical gray matter volume peaks at around age 11 in females and 13 in males. The complementary mechanisms of overproduction / selective elimination allow the brain to specialize in response to environmental demands. Areas such as the prefrontal cortex - a key component of neural circuitry involved in judgment, impulse control, and long range planning - are particularly late to reach adult morphometry, continuing to undergo dynamic changes well into the 20’s. Subcortical gray matter structures involved in decision-making and reward circuitry undergo dramatic changes around the time of puberty.
White matter volumes increase throughout childhood and adolescence reflecting ongoing myelination allowing greater “connectivity” and integration of neural circuitry from disparate parts of the brain. This increased coordination of brain activity is a hallmark of maturation, and is accompanied by an age-related increase in the correlation of activities in different parts of the brain on a wide variety of cognitive tasks. A tradeoff for the increased connectivity is that myelin releases molecules that impede arborization of new connections and thus decrease plasticity [4
These features of prolonged plasticity (but late maturation) of prefrontal (and other high association regions which integrate information from many parts of the brain), revamping of reward circuitry that guides decision making, and increasing connectivity of neural networks all support the adolescent brain’s fundamental mission of optimizing adaptation to its environment.
The link between adolescent brain evolution and the digital revolution does not in lie in a selection pressure wherein those with greater capacity to handle the demands of the technological changes have greater reproductive success. Even if that proved true, it would take many generations to have an evolutionary effect in that sense. The link lies in the evolutionary history that has made the human adolescent brain so adaptable.
With these principles in mind let us examine the neurobiology-environment interaction of the digital revolution with respect to the domains of education, entertainment, and social interactions.