Welcome to Your Child's Brain: How the Mind Grows From Conception to College (15 page)

Parents who try to keep their sons from playing with toy guns often discover that any stick—or, in a pinch, even a doll—can be converted to a weapon in a boy’s imagination.

Sex differences in behavior give girls a medium-sized advantage over boys in the classroom, where girls get better grades in high school and college. Girls’ brains mature earlier than boys’ brains, with the peak volumes of most brain structures occurring one to three years sooner in girls. Girls are moderately better at inhibitory control (d' of 0.4)—that is, sitting still and concentrating on their task—so the classroom culture is more friendly to girls. On average, girls are a bit more advanced in some areas of verbal development when starting school. Boys lag at fine motor coordination (d' of 0.6), giving them a moderate disadvantage in the ability to write letters, the largest sex difference in academic performance as school begins. These gaps persist through high school, with boys continuing to score lower on tests of both reading and writing.

But let’s put these gender comparisons in perspective. All these differences have smaller effects than the difference between living in a middle-class neighborhood with good schools (judged by their average test scores) and living in a low-income neighborhood with poor schools. First graders from poor areas score lower than their middle-class counterparts (d' greater than 1.1) in both reading and mathematics performance, and those gaps too typically widen with age (see
chapter 30
).

Like other gender differences, gender-related differences in education may be modified by experience. Girls have recently caught up with boys in academic areas where they were lagging just a decade or two ago. In the U.S., there are no remaining gender differences in average performance on mathematical achievement tests through high school. In addition, women are now more likely than men to attend and complete college. In the U.S., there are 185 women for every 100 men with college degrees at age twenty-two. Because men take longer than women to graduate, this gap narrows considerably at later ages, but it does not close completely.

To help reduce this gap, we suggest that boys might benefit from extra training in language and study skills during the early school years. The most efficient way to improve overall performance would be to provide such help evenhandedly to all children who need it, a group that would include more boys than girls.

Curiously, in the face of this female progress, the famous male advantage on the mathematics section of the SAT has not narrowed at all (d' = 0.4, thirty-five points as of 2009). Despite their lower average SAT scores, however, women get better grades than men in college math classes. One study found that male freshmen get the same grades in college math classes as women whose math SAT scores are thirty-five points lower. Indeed, nearly all the standardized tests required for college admission underpredict the future grades of women. This poor prediction may be due to better study habits among women (giving them higher grades for the same aptitude), or it may be due to gender bias in the SAT (giving women lower scores for the same aptitude). Either way, the lower grades of college men relative to women can be attributed in part to the use of standardized tests for admission decisions.

Another well-known group of sex differences falls in the realm of emotional behavior. These differences are not as large as most people believe. Effect sizes range from small to medium. These differences do not predict individual behavior very well, but some of them are noticeable at the group level. Girls are more likely than boys to express fear and to cry, but in both sexes the physiological responses to distress are similar. Many differences are so small that they are drowned out by individual variability within each sex. One example is the idea that boys make moral decisions based on justice (d' = 0.19), while girls make moral decisions based on relationships (d' = 0.28). Similarly, girls are only slightly better than boys at identifying emotions in other people’s faces (d' = 0.19 in childhood and
adolescence). Boys are only slightly more likely than girls to take risks at all ages (d' = 0.13), with a larger effect between ages ten and thirteen, and between ages eighteen and twenty-one (d' around 0.25). The gender gap in risk taking seems to be closing over time, as it was smaller in later studies (1980s and 1990s) than in earlier studies (1960s and 1970s).

Kids return to mixed-sex socializing as teenagers. The hormones of puberty usher in a medium-sized sex difference (d' = 0.53): 70 percent of teenage boys masturbate more often than the average teenage girl, a pattern that continues into adulthood. The size of this difference has declined from a d' of 0.96 over the past two decades, suggesting a strong cultural influence. Sex differences in self-esteem peak in adolescence as well, with teenage girls showing lower self-esteem than teenage boys (d' of 0.33, another small difference).

One area where girls could clearly use extra support is body image. Especially as teenagers, girls experience much more dissatisfaction than boys with their bodies, which is a risk factor for eating disorders and depression. The size of this difference increased from a small d' of 0.27 in the 1970s to a moderate d' of 0.58 in the 1990s, perhaps because of the progressively thinner standards of female beauty across recent decades.

Even if you’re concerned about your daughter’s weight, criticizing her body is likely to be counterproductive. In one
longitudinal study
(in which the same people are followed over a period of time), teenage girls and boys who reported being teased about their weight by family members were much more likely than average to have developed an eating disorder or to have become overweight five years later. Similarly, in another longitudinal study, repeated dieting in fourteen-year-old girls (many of whom were not overweight when the study began) increased their risk of becoming overweight a year later by almost a factor of five.

One interesting thing about all of these sex differences is that the size of the difference does not predict how malleable it is. Though initial preferences can be modified by environmental influences, they often do launch boys and girls onto paths that can lead them to have different experiences for much of childhood. To sum it up, your child probably has some initial inclination toward fixing cars, taking care of babies, or whatever, but there are many opportunities to broaden your child’s horizons by introducing him or her to new interests.

Chapter 9
ADOLESCENCE: IT’S NOT JUST ABOUT SEX

AGES: TWELVE YEARS TO TWENTY YEARS

You might dread your child’s adolescence, fearing a tumultuous period dominated by hormones and erratic behavior. But the truth is far more complex and includes many other changes, which are overwhelmingly for the good.

Although key steps in sexual maturation do occur during this time, a host of changes unrelated to sex also take place before and after puberty. More than anything else, the adolescent brain is highly dynamic. During adolescence, which begins with the onset of puberty, usually between ages eleven and thirteen, and continues until twenty and sometimes beyond, children make major moves toward living on their own. They explore new interests, organize their own behavior, and pursue serious relationships outside the family. They revel in (or feel awkward about) their bodies’ new capabilities. Most people recall their teen years as a time of near limitless possibility, of idealism, and of innumerable options. Friends of ours often find their early teenage daughter up late studying Spanish verbs, working on an intricate and beautiful drawing, looking up song lyrics, doing a conditioning regimen for her circus aerials, or simply reading or thinking. Whew.

Adolescence is also a time of risk. Just as developmental events before and after birth can lead to disorders such as autism, other problems are likely to become apparent during adolescence. Depression, bipolar disorder, drug addiction, and schizophrenia become increasingly prevalent at this time. In addition, adolescents
are prone to take risks because their sensation-seeking impulses become strong when self-regulation is not yet fully mature.

To superficial appearances, the brain appears to be nearly finished as children enter adolescence. By late childhood, the brain has reached 95 percent of its adult volume. Individual components are within 10 percent of adult size (some larger, others smaller). Behind this apparent maturity, though, some large changes are stirring.

The adolescent brain undergoes considerable reorganization as synapses are pruned away, continuing the process that began in childhood. The brain contains its maximum number of synapses (the connections between neurons) well before puberty, in people as well as other primates (see
chapter 5
). Studies of brain glucose consumption in children, as well as detailed counts of synapses, show that by early adolescence, the human neocortex has reached adult synapse numbers and uses about one fourth less energy than it did in early childhood. Even so, synapse elimination is far from complete. Indeed, measurements from rhesus monkeys show that their brains lose as many as thirty thousand synapses per second during adolescence. In our larger brains, the number is probably higher.

Before getting into what your adolescent’s brain is up to, let’s get technical for a bit. To explain how and why your child’s behavior is changing, we need to give you some details at the level of cells and connections that will provide essential context.

As you might expect, the changes in synapse number are accompanied by visible changes in the gray matter of the neocortex, where neurons, dendrites, and synapses are found. The general pattern is for gray matter to reach a peak thickness and then decline by 5–10 percent. In this way, the brain’s circuits are shaped and refined before adulthood—while the brain’s owner is acquiring new abilities to contribute more actively to the process.

These maturational changes happen at different times in different parts of the brain. Overall, the gray matter, containing all the neurons and synapses as well as a great deal of wiring, reaches peak volume by age nine to eleven. During this time, the
white matter
is still growing. In the neocortex, the first areas to reach peak thickness are the most extremely frontal and occipital (farthest back) regions. The areas in between are then filled in, starting from the back and moving forward. The temporal cortex reaches maximum thickness around age fourteen, followed by most of the frontal cortex. Finally, the white matter, made of myelinated axons that carry long-distance information, bulks up, especially connections between frontal and temporal cortex.

One sign that adolescent brains are becoming more efficient is that activity is better coordinated between distant brain areas. This improvement is seen in signals varying together (coherency) and traveling over distances more quickly. White matter is only 85 percent of adult size and continues to grow even into the forties. As white matter grows, axonal fibers are likely to be widening, and fatter axons transmit signals at higher speeds. Because white-matter axons mediate communication between distant brain regions, this change is likely to have
strong functional implications—though at present we don’t know what they are.

The tempo of developmental change varies from child to child. In a study of children whose brains were imaged repeatedly as they passed through late childhood and adolescence, children of higher intelligence had gray matter thickness that rose to peak more steeply—and declined more quickly as well. This result suggests the possibility that a key to intelligence is not brain size but capacity for change, though these differences are too variable for evaluating individuals. Indeed, increases and decreases in gray matter thickness also appear in childhood-onset schizophrenia and ADHD, so these structural changes may reflect a variety of underlying processes in different children.

To superficial appearances, the adolescent brain appears to be nearly finished. In fact, it is undergoing considerable reorganization.

What do all these brain changes mean for your adolescent’s thought—or lack thereof, as the case may be? The relatively late maturation of the frontal cortex has received a lot of media attention recently as a way of explaining adolescent impulsivity. Even one car insurance advertisement points out that this brain region is not done growing. This area participates in
executive function
tasks, such as self-control, planning, and resisting temptation (see
chapter 13
). It becomes more active with age, an exception to the general trend of decreasing activity. In anterior and superior regions of the frontal cortex, activity rises from ages twelve to thirty. In combination with the earlier maturation of
subcortical
areas participating in emotion and reward, adolescence is a time when the balance between impulse and restraint may be quite different from either childhood or adulthood.