In modern human populations, the size difference between males and females is not pronounced. In cultures where marriage and child bearing are delayed until women are fully mature, woman grow on average to about 80% the size of males. In modern humans, sexual differences express themselves in a variety of other, subtler ways, and it is often very difficult to distinguish differences that are sexual in nature (i.e., those grounded in biology) from those that reflect gender (that is, differences that are cultural in origin).

The biological differences between the sexes in modern humans (besides the obvious ones — genitalia, females' ability to bear offspring and nurse them) include

• different rates of maturation (females mature more quickly than males),
• aging (females experience menopause),
• longevity (females tend to live longer than males).

Males tend to have greater upper body strength and different kinds of coordination and seem specialized for short, intense bursts of effort. Women have greater stamina and more efficient hearts and lungs. And so on. 

Human cultures have usually imposed important values on these differences and elaborated their significance or meaning. Various kinds of ceremonies, often very elaborate in nature, mark the separate life stages of both males and females. 

At left, a male Xingu Indian from Brazil leads two young women who have been kept in seclusion nearly two years as part of a ceremony connected with the onset of menstruation. While this is an extreme case of cultural elaboration of physiological differences between males and females, it illustrates the point.