Most often, women are physically smaller — they weigh less and have smaller organs, less muscle mass, and more body fat — than men. Women also produce less of certain chemicals, such as those that synthesize the brain chemical serotonin (which plays a big role in depression and other mood disorders, as well as appetite and eating habits).

While men and women perform equally on intelligence tests, women's brains have more gray matter (the part that allows thinking) and less white matter (the part that transfers information among various regions). This may explain why women seem to be better at verbal and memory challenges, while men excel at spatial tasks.

Many of the differences in health concerns can be traced to hormones. For example, research has shown that female sex hormones might be related to the development and progression of allergies and asthma, both of which are more prevalent in women. Estrogen, the best known of the women's hormones, has been linked to women's greater susceptibility to lung cancer as well as the “hormonal cancers” that include breast and ovarian cancer.

Hormones can partly be blamed for women's greater sensitivity to pain and to stimulants like cocaine and amphetamines. This sensitivity also fluctuates with a woman's menstrual cycle. Hormones might explain why women often react differently than men to anesthesia (women wake up more quickly but are more prone to side effects such as postoperative vomiting or nausea).