Although the words are sometimes used interchangeably, sex refers to the biological aspects of being male and female, while gender refers to the social construction by which we define maleness and femaleness.

We tend to emphasize gender differences. But there's been a focus on gender similarities. Men and women, boys and girls, in their thinking are much more similar than they're different.

Your opposite sex is, in reality, your very similar sex. And should we be surprised? Of the 46 chromosomes, men and women have 45 in common.

That being said, there are some differences, sex differences, some of which emerge quite early.

Males and females are variations on a single form. Seven weeks after conception, you were indistinguishable from someone of the other sex. Then genes activate biological sex, which is determined by your 23rd pair of chromosomes, the two sex chromosomes.

The X chromosome is received from the mother. From one's father, either another X chromosome making you a girl, or a Y chromosome making you a boy. But sometimes during prenatal development, genetic abnormalities or hormone levels can affect how genitalia or reproductive organs develop. Some individuals might develop combinations of physical features from both sexes. These people are called intersex.

Another key period for sexual differentiation falls during the fourth and fifth prenatal months, when sex hormones bathe the fetal brain and influence its wiring, especially in brain areas with abundant sex hormone receptors. In adulthood, parts of the frontal lobes, an area involved in verbal fluency, are reportedly thicker in women. Part of the parietal lobe involved in space perception is reportedly thicker in men. Other studies report gender differences in the hippocampus, the amygdala, and the volume of brain gray matter versus white matter.

Despite these apparent biological differences in sex, it is clear that the society we are born into deeply affects our gender. When we enter the world, it is less clear how hard wired sex differences impact our behavior.

There are some consistent gender differences that get talked about a lot, for which there seems to be replicable evidence. And these differences get picked up, both by parents and by educators. And then they color our expectations of children and their capacities and their talents.

For example, reasoning about three dimensional solid forms. It's not that men are good and women are bad. But men and women may have very different strategies, and those strategies may begin to emerge quite early, age five or six or something like that. So men are more holistic and tend to rotate things in their head as a block, and women are more strategic. And we need to look at this, as I said with the cultural thing, different ways of solving problems that lead to perhaps the same solution or alternative solutions that are just as good.

So if you want to understand why we see so many boys going into math and science and engineering as compared to girls, the answer is probably not going to be that the girls don't have the stuff that it takes fundamentally that gets them prepared to do that kind of quick learning. But rather, it's going to be part of that intricate dance between the culture and the individual.

At your birth, everyone wanted to know boy or girl? We humans share an irresistible urge to organize our worlds into simple categories. When dressed identically though, adults rarely can tell apart male and female infants. In many ways, children's behavior mirrors the gender role stereotypes that are predominant in our culture.

Partly it's a matter of how we define the construct. So I'm always a little bit wary of sex differences, because we tend to find what we think we're looking for.

Children are gender detectives. Once they grasp that two sorts of people exist and that they are of one sort, they search for clues about gender. And they find them in language, dress, toys, and songs.

The kinds of toys that boys play with tend to be more active, more engaging, require more physical manipulation. And that starts almost as soon as kids can hold things.

If you look at parental preferences for toys, I think they're still pretty gender typed. Fathers are still troubled, for example, if their son plays with a feminine looking doll. And its just even modern day dads who would immediately eschew any sexists beliefs, behaviorally would still find that hard to deal with.

I mean, I know there are exceptions to that. But there are these cultural beliefs that sometimes we don't realize how much of a strong influence that they make.

If their genes and hormones do predispose males to be more physically aggressive than females, culture may magnify this gender difference through norms that encourage males to be macho and females to be the kinder, gentler sex. If men are encouraged toward roles that demand physical power and women toward more nurturing roles, each may then exhibit the actions expected of those who fill such roles and find themselves shaped accordingly. But gender roles are converging. Brute strength has become increasingly irrelevant to power and status.

I think we tend to overemphasize gender difference. Men are not from Mars and women from Venus. They're from the very same planet, and they're much more similar than they're different.

And as women's employment in formerly male occupations has increased, gender differences in traditional masculinity or femininity have diminished. As the roles we play change over time, we change with them.