Biological Influences Starting in the preschool years, boys and girls congregate primarily with other children of their sex, and most prefer the toys and games of their own sex. They will play together if required to, but given their druthers, they usually choose to play with same-sex friends. The kind of play that young girls and boys enjoy also differs, on average. Little boys, like young males in all primate species, are more likely than females to go in for physical roughhousing, risk taking, and aggressive displays. These sex differences occur all over the world, almost regardless of whether adults encourage boys and girls to play together or separate them (Lytton & Romney, 1991; Maccoby, 1998, 2002). Many parents lament that although they try to give their children the same toys, it makes no difference; their sons want trucks and guns and their daughters want dolls.

Biological scientists believe that these play and toy preferences have a basis in prenatal hormones, particularly the presence or absence of prenatal androgens (masculinizing hormones). Girls who were exposed to higher-than-normal prenatal androgens in the womb are later more likely than nonexposed girls to prefer “boys' toys” such as cars and fire engines, and they are also more physically aggressive than other girls (Berenbaum & Bailey, 2003; Martin & Dinella, 2012). A study of more than 200 healthy children in the general population also found a relationship between fetal testosterone and play styles. (Testosterone is produced in fetuses of both sexes, although it is higher on average in males.) The higher the levels of fetal testosterone, as measured in the amniotic fluid of the children's mothers during pregnancy, the higher the children's later scores on a measure of male-typical play (Auyeung et al., 2009). In studies of rhesus monkeys, who of course are not influenced by their parents' possible gender biases, male monkeys, like human boys, consistently and strongly prefer to play with wheeled toys rather than cuddly plush toys, whereas female monkeys, like human girls, are more varied in their toy preferences (Hassett, Siebert, & Wallen, 2008).

Do these findings have anything to do with gender identity, the core sense of being female or male? A psychologist reviewed hundreds of cases of children whose sex of rearing was discrepant with their anatomical or genetic sex. He learned that the answer is enormously complex because a person's gender identity depends on the interactions of genes, prenatal hormones, anatomical structures, and experiences in life (Zucker, 1999). Consider a longitudinal study of 16 genetic males who had a rare condition that caused them to be born without a penis. The babies were otherwise normal males, with testicles and appropriate androgen levels. Two of the boys were raised as male and developed a male gender identity. Fourteen had been socially and surgically assigned to the female sex, according to the custom when they were born. Of those 14, eight eventually declared themselves male, five decided to live as females, and one had an unclear gender identity (Reiner & Gearheart, 2004).

Cognitive Influences Cognitive psychologists explain the mystery of children's gender segregation and toy and play preferences by studying children's changing cognitive abilities. Even before babies can speak, they can discriminate two sexes. By the age of 9 months, most babies can discriminate female from male faces (Fagot & Leinbach, 1993), and they can match female faces with female voices (Poulin-Dubois et al., 1994). By the age of 18 to 20 months, most toddlers have a concept of gender labels. They can accurately identify their own gender and that of people in picture books and begin correctly using the words boy, girl, and man (interestingly, lady and woman come later) (Zosuls et al., 2009).

After children can label themselves and others consistently as being a boy or a girl, shortly before age 2, they change their behavior to conform to the category they belong to. Many begin to prefer same-sex playmates and sex-traditional toys without being explicitly taught to do so (Halim et al., 2014; Martin, Ruble, & Szkrybalo, 2002). They become more gender typed in their toy play, games, aggressiveness, and verbal skills than children who still cannot consistently label males and females. Most notably, girls stop behaving aggressively (Fagot, 1993). It is as if they go along behaving like boys until they know they are girls. At that moment, they seem to decide: “Girls don't do this. I'm a girl; I'd better not either.”

By about age 5, most children have developed a stable gender identity, a sense of themselves as being male or female regardless of what they wear or how they behave. Only then do they understand that what girls and boys do does not necessarily indicate what sex they are: A girl remains a girl even if she can climb a tree, and a boy remains a boy even if he has long hair. At this age, children consolidate their knowledge, with all of its mistakes and misconceptions, into a gender schema, a mental network of beliefs and expectations about what it means to be male or female and about what each sex is supposed to wear, do, feel, and think (Bem, 1993; Martin & Ruble, 2004). Gender schemas are most rigid between ages 5 and 7; at this age, it's really hard to dislodge a child's notion of what boys and girls can do (Martin et al., 2002).

Gender schemas even include metaphors. After age 4, children of both sexes will usually say that rough, spiky, black, or mechanical things are male and that soft, pink, fuzzy, or flowery things are female; that black bears are male and pink poodles are female (Leinbach, Hort, & Fagot, 1997; Swinkels, 2009). But the content of these schemas is not innate. A hundred years ago, an article in Ladies' Home Journal advised: “The generally accepted rule is pink for the boys, and blue for the girls. The reason is that pink, being a more decided and stronger color, is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl” (Paoletti, 2012). The invention of gender-specific “rules” of clothing for babies and young children was a creation of late 20th-century marketing.

Many people retain inflexible gender schemas throughout their lives. They feel uncomfortable or angry with men or women who break out of traditional roles, not to mention with transgendered individuals who don't fit either category or want to change the one they grew up with. However, with experience and cognitive sophistication, older children often become more flexible in their gender schemas, especially if they have friends of the other sex and if their families and cultures encourage such flexibility (Martin & Ruble, 2004).

Cultures and religions, too, differ in their schemas for the roles of women and men. In all Western, industrialized nations, it is taken for granted that women and men alike should be educated; indeed, laws mandate a minimum education for both sexes. But in cultures where female education is prohibited in the name of religious law, many girls who attend school receive death threats and some have had acid thrown on their faces. Gender schemas can be powerful, and events that challenge their legitimacy can be enormously threatening.

Learning Influences A third influence on gender development is the environment, which is full of subtle and not-so-subtle messages about what girls and boys are supposed to do. Behavioral and social-cognitive learning theorists study how the process of gender socialization instills these messages in children (Bussey & Bandura, 1999). They find that gender socialization begins at the moment of birth. Many parents are careful to dress their baby in outfits they consider to be the correct color and pattern for his or her sex. Clothes don't matter to the infant, of course, but they are signals to adults about how to treat the child; remember the case of Casey, whom you read about earlier? Adults often respond to the same baby differently, depending on whether the child is dressed as a boy or a girl (Shakin, Shakin, & Sternglanz, 1985).

Parents, teachers, and other adults convey their beliefs and expectations about gender even when they are entirely unaware that they are doing so. When parents believe that boys are naturally better at math or sports and that girls are naturally better at English, they unwittingly communicate those beliefs by how they respond to a child's success or failure. They may tell a son who did well in math, “You're a natural math whiz, Johnny!” But if a daughter gets good grades, they may say, “Wow, you really worked hard in math, Joanie, and it shows!” The implication is that girls have to try hard but boys have a natural gift. Messages like these are not lost on children. Both sexes tend to lose interest in activities that are supposedly not natural for them, even when they all start out with equal abilities (Dweck, 2006; Frome & Eccles, 1998).

In today's fast-moving world, however, society's messages to women and men, and parents' messages to their children, keep evolving. As a result, gender development has become a lifelong process, in which gender schemas, attitudes, and behavior change as people have new experiences and as society itself changes (Rosin, 2012). Five-year-old children may behave like sexist piglets while they are trying to figure out what it means to be male or female. Their behavior is shaped by a combination of hormones, genetics, cognitive schemas, parental and social lessons, religious and cultural customs, and experiences. But their gender-typed behavior as 5-year-olds usually has little to do with how they will behave at 25 or 45. In fact, by early adulthood, women and men show virtually no average differences in cognitive abilities, personality traits, self-esteem, or psychological well-being (Hyde, 2007).

That is why children can grow up in an extremely gender-typed family and yet, as adults, find themselves in careers or relationships or identities they would never have imagined for themselves. If 5-year-olds are the gender police, many adults end up breaking the law.