According to a 19th century nursery rhyme, the biological distinctions between males and females are thus:

What are little boys made of?
What are little boys made of? 
Frogs and snails
And puppy-dogs' tails,
That's what little boys are made of.
What are little girls made of?
What are little girls made of? 
Sugar and spice
And all things nice,
That's what little girls are made of.

This was pre-Mendel but we bet he agreed.    Boys seem to really get cheated on cute rhymes about gender differences.   But what came first, the rhyme or the differences?   

A new longitudinal study by researchers at The Pennsylvania State University, the University of Hawaii at Manoa, and Purdue University published in the March/April 2009 issue of Child Development says that sex-typed characteristics develop differently in girls and boys.

The researchers looked at first- and second-born siblings from nearly 200 mostly White, middle-class American families. They collected information through home interviews conducted over seven years, activity diaries provided by the children, and saliva samples that measured the children's testosterone levels.

Not surprisingly, girls and boys differed in their sex-typed personality qualities and their sex-typed activity interests in early adolescence, with girls showing higher levels of expressive traits (such as kindness and sensitivity) and interest in "feminine" activities (such as the arts and reading), and boys showing higher levels of instrumental traits (such as independence and adventurousness) and interest in "masculine" activities (such as sports and math).

Girls' stereotypically feminine, expressive traits didn't change over time. In contrast, boys' sensitivity and warmth declined substantially across middle childhood but increased in later adolescence so that by about age 19, boys reported about the same levels of sensitivity and warmth as girls. For stereotypically masculine traits such as independence and adventurousness, girls showed increases only in middle childhood, but in boys, these traits rose across adolescence. This pattern meant that by the end of high school, boys had many more of these characteristics than girls.

The study also found that changes in girls' and boys' personality traits and interests were related to how they spent their time. In general, girls who spent time with other females developed female personality characteristics, and boys who pursued activities with other males developed male characteristics. Time with female peers was the exception: Boys and girls who spent time with friends who were girls increased in independence and adventurousness.

The research also found that interests and traits developed differently in first-born children than in children born second. For example, second-born children showed increases in traits like adventurousness and independence across adolescence, whereas in firstborns, these traits did not change much over time. These findings are consistent with the idea that first-borns conform more, while second-borns are more likely to rebel. Finally, children who showed faster rates of increase in the hormone testosterone in early adolescence weren't as affected by social influences on their personality development.

The study was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development.