The Teenage Brain: A Neuroscientist's Survival Guide to Raising Adolescents and Young Adults

Jensen describes receiving correspondence from desperate parents. Letters and emails frequently recount that a well-behaved child has transformed into a difficult, unrecognizable teen. The author explains there are neurological reasons behind these changes. As adolescents’ brains are undergoing considerable restructuring, they are not fully to blame for difficult behavior.

Adolescence was only recognized as an individual development state in the mid-20th century. Up to that point, teenagers were treated as small adults. During the 19th century, American teenagers worked like adults—first on the land and then in factories. In 1904, the groundbreaking American psychologist Granville Stanley Hall identified adolescence “as a discrete developmental stage” that began with puberty (17). However, it was from 1929 that attitudes toward teenagers began to change. A scarcity of jobs during the Great Depression led to a reduction in child labor in the United States. Teenagers increasingly went to high school and were financially dependent on their parents until they left. By the 1940s and 1950s, the teenager emerged as “a new culture” (17).

The author argues that, even today, the line between adolescence and adulthood remains indistinct. Finishing high school is often considered to be the end of adolescence. However, the laws of the United States give no clear definition of the age that an individual becomes a responsible adult. While 16 is the legal age to drive in many states, teenagers must be 18 to vote, buy cigarettes, and join the military. Alcohol cannot be legally consumed until the age of 21.

Jensen asserts that the tendency to attribute all difficult teenage behavior to “hormones” is a gross oversimplification. She explains that hormones are present in the body from birth, but at puberty the concentration of sex hormones (testosterone, estrogen, and progesterone) substantially changes. These physical changes are accompanied by alterations in the brain chemicals that control mood. Teenagers’ brains react differently to hormones than those of adults. Sex hormones have a potent effect on the limbic system, creating a more intense emotional response. Increased sex hormones combine with immature frontal lobes that are unable to modulate them, meaning teenagers are likely to pursue “emotionally charged experiences” (20).

Jensen clarifies that hormones only partly explain why teenagers are emotionally volatile, take risks, and exhibit poor judgment. A second factor is the role of neurotransmitters, described by the author as the brain’s “messengers.” Neurotransmitters are still developing and changing during adolescence. This makes the brain flexible and open to new ideas but also means teenagers are particularly vulnerable to stress, alcohol, and drugs. 

In this chapter, Jensen describes the human brain’s complex structure. The skull protects the brain, which has two hemispheres. The left hemisphere controls the right side of the body, and the right hemisphere controls the left. The brain’s outer layer—the cortex—consists of gray matter. Its surface is folded into ridges and fissures known as gyri and sulci. These folds developed to fit as much brain matter into the human skull as possible. In simpler species, such as cats and dogs, the brain has a relatively smooth surface.

The gray matter of the brain is made up of cells called neurons. Neurons connect with one another via the brain’s white matter, which is coated with an insulating material called myelin. Neurons control emotions, behavior, and bodily functions. The complexity of the neuron network, rather than the size or weight of a brain, determines intelligence. For example, Jensen reveals that Albert Einstein’s brain was slightly lighter than average but displayed more complex connections than usual. Female brains are smaller than males’ but display the same IQ range.

In the early 20th century, Canadian neuroscientist Wilder Penfield developed the homunculus: a map of the brain and its corresponding body parts. Penfield created the homunculus by stimulating areas of the brain to see which part of the body they affected. He discovered that the space allocated to certain body areas in the brain corresponds to the complexity of their function. For example, the face and fingertips take up relatively large areas, while the area for the back is small.

Jensen introduces the concept of brain plasticity. She explains that areas of the brain can become larger or smaller depending on frequency of use and experience. After basing her undergraduate thesis on brain plasticity, the author demonstrated the concept at home with a pet kitten. Jensen and her family regularly massaged the kitten’s paws to develop the associated brain area. As a result, the kitten became more “paw-centric,” using her paws for activities such as eating. The kitten favored her left paw, as this was the side stimulated the most.

The brain is divided into four lobes: frontal (responsible for “executive function”), parietal (controlling movement and sensation), temporal (controlling language, emotion, and sexuality), and occipital (responsible for vision). Located above the brainstem and spinal cord, the frontal lobes make up 40% of the brain and are the last to mature. In most animal species, the frontal lobes are comparatively small, while other areas are more developed. For example, dolphins have a larger and more sophisticated auditory cortex.

Functional magnetic resonance imaging (fMRI) scans show the connections between different areas of the brain and how they activate one another. A key difference in the adolescent brain is its “internal wiring.” Teenagers have an excess of gray matter and a deficiency of white matter—the material that speeds up neural messages sent from one area of the brain to another. Scans also reveal that the neural connections to the frontal lobes are the last to be developed and do not fully mature until the age of 20 or older. Jensen states that, as the teenage brain is only 80% developed, the missing 20% explains much about teenage behavior. The frontal lobes affect decision-making and judgment by providing insight into possible risks and consequences.

The author provides readers with an example of the impact of immature frontal lobes. She cites the case of a college student who drowned in a tennis club pool after breaking in while drunk. Revealing that she immediately told her sons about the incident, Jensen emphasizes the importance of talking to teenagers about such potential dangers. Adolescents’ underdeveloped frontal lobes require repeated reminders of risk in order to remember information for the future.

Also late to develop are the parietal lobes, which are responsible for switching between tasks. Jensen claims that multitasking is “a myth,” as the brain cannot remain focused on two things at the same time. Nevertheless, young people generally believe they are skilled at multitasking. This misconception is detrimental to studying, as teenagers often do so with multiple background distractions, such as television, music, and texting. The author points out that teenagers’ attempts to multitask are particularly hazardous when driving. She reveals that 87% of adolescent deaths in car accidents are caused by drivers becoming distracted.

The brainstem is responsible for automatic bodily functions that require little or no conscious effort, such as breathing. Both the brainstem and the spinal cord are connected to the thalamus, which, in turn, links to the rest of the brain. The thalamus is part of the limbic system, which also contains the amygdala, the hippocampus, and the hypothalamus. The overall function of the limbic system is to assimilate memories and emotions. This part of the brain is a key factor in adolescent behavior.

The role of the hippocampus is to store and retrieve memories. Scientists first discovered the connection between this area of the brain and memory formation in 1953 after a man known as “H.M.” had brain surgery. During the operation, most of H.M.’s hippocampus was removed to reduce his incapacitating epileptic fits. The surgery was successful but left the patient unable to create new memories. H.M. could only remember events prior to the operation. Jensen explains that the hippocampus is stimulated by every new experience. In adolescents, this brain area is more active than in adults, which accounts for teenagers’ cravings for new experiences and their capacity to learn new information.

The amygdala in the limbic system is affected by hormones. Jensen explains that, in teenagers, this region is both underdeveloped and overexcitable and is believed to cause “adolescent explosiveness.” The author cites the example of a 16-year-old who tried to drive off in his parents’ car when told he had not earned the privilege of driving lessons. Incensed, the teenager forgot to open the garage door and drove straight into it.