SEX IS NOT AS SIMPLE as male or female. It’s not just about chromosomes. Or genitals. Or reproductive cells. Or any other binary metric. (Binary refers to something that can only take one of two, either/or states.)
When it comes to sex, millions of people don’t fit a narrow, binary definition. That’s not their fault and many don’t even know it. Instead, scientists say, sex should be viewed in all its complexity.
Sex is a multifaceted trait that has some components that are present at birth and some components that develop during puberty. And each of these components shows variation. This shows the challenge with trying to say someone is male or female based on just one trait, such as what reproductive cells (eggs or sperm) they can make. But it leaves out people who don’t make any reproductive cells.
Most traits ascribed to males and females fall along a spectrum that has two peaks. One is the average for females. The other is the average for males. On average, males are taller than females. Males also, on average, have more muscle mass, more red blood cells and a higher metabolism. A definition of biological sex based just on gametes misses all that complexity.
Scientists used to think that embryos automatically developed as female unless there were specific instructions to become male. But researchers have learned that’s not true. To develop as females, embryos must actively dismantle male-producing structures. And they have to build ones that support female reproduction.
About six weeks into gestation, cells appear that will eventually give rise to ovaries (the tissue that makes eggs) or testes (the tissue that can make sperm). At about eight weeks into gestation, certain cells in what will become the testes begin to make testosterone. This hormone is important for development of the scrotum and penis and other male reproductive organs. But males don’t make sperm at this point. That’s because testosterone levels dip late in pregnancy. And they stay low until puberty. Then testosterone kicks in once more. At this point, immature cells can morph into sperm.
Ovaries don’t produce any sex hormones in the fetus. And the uterus, fallopian tubes and vagina develop without any input from hormones. Females generally have two X chromosomes. Males typically have an X and a Y. But there are plenty of variations on that theme.
About 1 in every 650 male babies are born with two or more X chromosomes and one Y. These people have testes and penises. They don’t, however, make sperm. So a definition based on gametes would leave these people out.
With so much natural variation among people, trying to define sex by any single factor creates confusion. Plus, unless they need it for medical reasons, most people never have their gametes or chromosomes checked. So neither is a practical way to define whether someone is male or female. By Manny Palomar, PhD (EV Mail OCTOBER 13-19, 2025 Issue)
