Gender identityChildren begin to identify themselves as a boy or a girl as early as 18 months. But it's between the ages of 2 and 6 that they begin to identify with others of their sex and demonstrate play and other behaviors that are characteristic of that sex. Most social scientists agree that an interplay of nature and nurture determines how these gender roles play out.
Between the big growth stages of infancy and adolescence, boys and girls grow in height and weight at about the same slow-but-steady rate. There aren't notable differences between the sexes until late elementary school, when girls start to grow taller faster, although boys catch up and exceed them within a few years.
Boys' gross motor skills (running, jumping, balancing) tend to develop slightly faster, while girls' fine motor skills (holding a pencil, writing) improve first. Often girls show an interest in art (painting, coloring, crafts) before boys for this reason.
Boys are also more physically aggressive and impulsive, as revealed by studies of their brains. The pleasure center of the brain actually lights up more for boys when they take risks. That's not to say that girls aren't active and risk-taking, only that on average boys are more so.
More boys than girls are late talkers, and boys use more limited vocabularies. Girls are better at reading nonverbal signs, like tone of voice and expression, which also makes them better communicators early on, as they can connect feelings and words faster.
This is something you can focus on when reading books with boys: Point out characters' emotions, so boys start to notice how others are feeling.
Girls are potty-trained earlier than boys on average, though it's unclear whether this is due to physical differences or differences in socialization. (Mom usually does the training and may be easier for a girl to identify with.) Fewer girls wet the bed, too.