Humans

Biological sex is not as simple as male or female

Defining sex as a binary excludes many biological realities, scientists say

A conceptual illustration representing the development of biological sex. The art is abstract and features shades of purple, orange and yellow. Two cells are splitting with a pair of human hands appearing to grab either end of the cells. A human silhouette is in the middle of the art, with developing faces surrounding it. There are DNA strands on the edges of the art that end in human hands.

The biology of sex is complicated.

Rebekka Dunlap

By Tina Hesman Saey

Sex is messy.

It’s not just about chromosomes. Or reproductive cells. Or any other binary metric. Many genetic, environmental and developmental variations can produce what are thought of as masculine and feminine traits in the same person. And so sex, scientists say, should be viewed in all its complex glory.

“Sex is a multifaceted trait that has some components that are present at birth and some components that developed during puberty, and each of these components shows variation,” says Sam Sharpe, an evolutionary biologist at Kansas State University in Manhattan.