Difference in Context Boys and girls have a lot in common but they also differ. They begin their developmental journeys with different chromosomes, but at first have identical gonads, and internal and external genitals. Furthermore, the fact that the differences involved are so small means that the brain size of most boys and girls are not different at all. Data from the 1970s and 1980s showed a 25 percent deficit in complex problem solving for girls compared to boys, starting in high school. At birth boys outweigh girls by almost half a pound , although the weight differences disappear by ages 2-3 years. Knowing the developmental dynamics of a trait allows researchers to look at the emergence or disappearance of difference, asking what components of each individual’s being and experience contribute to its growth and maintenance.