Welcome to Your Child's Brain: How the Mind Grows From Conception to College (14 page)

This idea is easier to explain in pictures than in words. The figure below shows the differences between groups that correspond to typical d' values. The horizontal axis represents the possible scores, while the height of the curve represents the number of people in the population who get a particular score. From top to bottom, these differences would be considered small, medium, and very large.

Let’s consider some specific examples. For gender differences in adult height in the U.S., the left curve in the bottom panel (d' = 1.9) would represent women, and the right curve would represent men. The horizontal axis would show heights from short (left) to tall (right), with the peak of the female curve at 5 feet 3.8 inches, the average height for women, and the peak of the male curve at 5 feet 9.4 inches, the average height for men. A man of average height is taller than 92 percent of women. In the research literature, a value of d' that is at least 0.8 is considered large, so this would be a very large difference.

At the other extreme, let’s take as our example a small difference that we’ve already mentioned: hearing. Several authors have recently argued in favor of single-sex education based in part on the idea that girls have more sensitive hearing than boys and therefore respond best to teachers speaking quietly. For hearing sensitivity, the left curve in the top panel (d' = 0.2) would represent boys, and the right curve would represent girls. Because the difference between the two groups is small, as you can see, the two curves overlap substantially.

The individual differences in hearing within each sex are much larger than the differences between boys and girls. And given that many boys have sharper hearing than many girls, it doesn’t make sense to argue for sex segregation on these grounds. If you think sensitive hearing affects the way people learn, you should separate them based on their hearing, not their gender—the two are not the same.

Only a few gender differences are big enough to predict individual behavior. The largest known behavioral difference at any age is toy preferences in three-year-olds. 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. Given the choice between a boy-typical toy like a car and a girl-typical toy like a tea set, at age three children differ in their choice of boyish toys with a d' of 1.9, a difference corresponding to the bottom panel of the figure. This means that you can do quite well at guessing the sex of a young child based on his or her choice of toys, as 97 percent of boys are more likely to play with male-typical toys than an average girl. Because play helps children learn and practice a variety of skills, sex differences in how children spend their time
can influence which abilities they carry through life (see
Practical tip: Broadening your child’s abilities
).

The emergence of toy preferences is an early stage in the development of
gender identity
, defined as your child’s self-identification as male or female. Gender-influenced toy preferences are seen across cultures, beginning around one year of age. Even babies have some understanding of gender (see
chapter 1
), but only a few two-year-old children can accurately state whether they are boys or girls or reliably distinguish men from women in pictures. Most children—again across cultures—reach this milestone by two and a half, and almost all children get there by age three. Children who have reached this milestone are less likely to choose the “wrong” toy than children who have not.

Toy preferences almost certainly have an innate basis (though they are also influenced by culture). One clue is that male monkeys prefer to play with trucks, while female monkeys prefer dolls. Another clue is that boy-typical toy preferences are more common in girls with a syndrome called
congenital
adrenal hyperplasia
or
CAH
. Due to a genetic defect in adrenal hormone synthesis, CAH girls are exposed to an excess of
testosterone
and other androgens, masculinizing their brains and to some extent their bodies in utero. Because this hormonal malfunction can be treated starting at birth, CAH girls offer an opportunity to look at the effects of prenatal exposure to male hormones on later behavior.

As they get older, girls tend to become more flexible in their toy preferences. By age five, nearly half will pick a boy-typical toy when offered a choice. Boys, on the other hand, continue to refuse girl-typical toys, most likely because the social penalty for acting like a girl is very steep. Both peers and parents—especially fathers—actively discourage boys from playing with girl toys.

Some parents are concerned that allowing their son to play with girl toys will lead him to be gay in adulthood, but this worry confuses correlation with causation. Whether parents encourage or approve of their son’s habits is irrelevant to his sexual orientation later in life. It is true that about half of the boys who prefer girl toys do grow up to be gay—and also true that many of them do not. (Tomboy girls, on the other hand, rarely turn out to be lesbians.) Playing with dolls doesn’t cause boys to become gay, though. The most likely explanation is that playing with girl toys and adult homosexuality both result from earlier influences on some boys’ brains, perhaps due to prenatal experiences or genetics. Psychiatric treatment aimed at encouraging boyish behavior has no effect on adult sexual orientation, but the father who tries to discourage girlish behavior might be opening a rift with his son. By the time you can observe the behavior, the outcome is out of your control, so you might as well get comfortable with it.

You’ve probably noticed two other sex differences in young children’s behavior. Boys are significantly more active and more physically aggressive than girls. These differences are medium sized, with a d' of 0.5, corresponding to the second panel of the
figure
. That means an average boy is more active and more physically aggressive than 69 percent of girls, which does not predict individual behavior very well, but does make groups of boys obviously different from groups of girls. These differences are also probably influenced by the action of hormones on the brain, not just by culture. Juvenile male monkeys show more rough-and-tumble play than female monkeys, and this behavior can be modified by hormone treatment. Similarly, CAH girls are more aggressive and more active than other girls, again suggesting an early hormonal influence on this behavior.

PRACTICAL TIP: BROADENING YOUR CHILD’S ABILITIES

Children’s play may affect their later behavior and interests. You can’t force boys to behave like girls or vice versa, but by taking your children’s natural inclinations into account, you can help them to practice skills that they might not find on their own. You don’t know what the future holds, and we figure that you can’t go wrong by increasing the number of options available to them in adulthood.

One of the largest adult sex differences is that males are better at mentally rotating objects through space. (This ability affects the way we think about directions, as well as some practical skills like moving a couch through a doorway.) This pattern emerges early in life and is then modified by later experience. Many male infants at three to five months can recognize rotated objects, while few female infants of the same age can do so. Otherwise infants show no sex differences in their understanding of the behavior of objects (see
chapter 1
). In elementary school, the gap in mental rotation ability is small, but it continues to widen as children mature, reaching a d' of 0.66 to 0.94 (depending on how the test is scored) for adults, meaning that the average man performs better than 75 to 83 percent of women. Performance on mental rotation tests predicts performance on the math part of the SAT (originally Scholastic Aptitude Test, later renamed Scholastic Assessment Test I) in both male and female high school students and likely contributes to sex differences in map reading and navigation ability.

It makes sense that different styles of play might improve different skills. Exploring physical objects and their interactions is an important component of boys’ play. As they build towers of blocks and knock them down, wrestle, play catch, or ride bikes around the neighborhood, boys are learning about the rules of the physical world. As girls play with dolls and dollhouses, they are practicing nurturing and fine motor control skills. Girls also talk with each other during play more than boys do, which may help girls to become more fluent and have larger expressive vocabularies by the time they start school.

Boy-style play develops spatial skills in all brains. Boys raised in deprived conditions don’t show an advantage over girls in their spatial abilities. In one study, boys from families with low socioeconomic status (SES; see
chapter 30
) scored lower on a mental rotation test than boys from medium- or high-SES families and performed no better than girls of any SES. Boys from such families may not get the play experiences required to develop their object manipulation skills. Video games involving navigation or other spatial tasks help boys and girls learn to visualize and rotate objects. Some studies find that these training effects are especially large in girls. Playing sports may also be helpful. College athletes of both sexes have an advantage over non-athletes in mental rotation tasks and other spatial skills, though this may be because people with good spatial abilities are more likely to play sports. Researchers have not yet demonstrated that these experiences lead to real-world improvements in spatial skills, but that will be the next step.

How can parents help all their children develop a broad range of abilities? Encouraging girls to play video games could improve their spatial reasoning (as well as their comfort with computers). We also suggest getting girls involved in sports (see
chapter 15
) when they’re young, as self-consciousness may inhibit teenage girls from wanting to learn new physical skills.

Parents can help boys to develop better language skills by talking and reading to their sons, starting in infancy (see
chapter 6
). Boys may also benefit from extra help with phonological awareness in the preschool years, which parents can provide by discussing which letters make which sounds as they read. Similarly, you can take advantage of young boys’ attraction to the computer to encourage them to write stories onscreen or choose books about fighter pilots or dinosaurs to engage their interest in reading. You can find many other suggestions for helping girls and boys grow into well-rounded adults in the neuroscientist Lise Eliot’s book
Pink Brain, Blue Brain
.

Perhaps as a consequence of these behavioral differences, starting as early as two or three years of age, children prefer to play with other children of the same sex. These segregated play groups persist throughout elementary school. This pattern is seen across many societies, from agricultural villages to big cities. It does not seem to depend on whether or not the adults in the society have strongly segregated sex roles, though cultural factors can modify the pattern’s expression somewhat. It even occurs in monkeys and apes. If there are only a few children available, boys and girls will play together, but when possible, they usually split the group by gender.

This behavior reinforces gender norms as children are learning about their
gender identities. Social pressure from other children to conform to gender norms is particularly strong from age four to eight, perhaps because children’s early concepts of sex roles (and many other rules of society) tend to be written in black and white, with a more flexible understanding emerging later in development. We know a neuroscientist couple whose son was best friends with a girl throughout the preschool years. When he was six, his male friends at summer camp let him know that playing with girls was unacceptable. Now their son will only see his female friend inside the house, with everyone involved sworn to secrecy so the other boys won’t find out. Single-sex groups often act in such a way to communicate and enforce gender-specific behaviors.