Trans Athletes and Women's Sports

Peter Aitken, Ph.D.

The author is retired from the faculty at Duke University Medical Center.

There has been much discussion recently about whether trans women—genetic males who have transitioned to the female gender—should be allowed to compete in women’s sports. Unfortunately, there is sometimes anti-trans bigotry involved on one side as well as reflexive, uninformed reactions from trans rights supporters on the other. Often lacking is sufficient awareness of the biological facts that are highly relevant to this conundrum. I have written this brief essay to help those who want to be aware of the biological and medical facts that are relevent to this debate.

In humans and other animals, biological sex is genetically determined by a single pair of chromosomes. Females have two X chromosomes and males have one X and one Y chromosome. These chromosomes were discovered in 1905 and can be seen under a microscope. And it’s these chromosomes that are responsible for development of sexual characteristics such as ovaries and a uterus in females and testes and a penis in males.

These chromosomes are also responsible for genetic males generating high levels of the hormone testosterone before birth, for a short period during their first year, and then, starting at puberty (age 12-13), for the rest of their lives (decreasing slowly with age). Genetic females do not have more than trivial levels of testosterone. The biological effects of testosterone are major and very well documented; they include increases in muscle mass, bone density, body size, and the oxygen carrying ability of blood—all factors that are directly related to athletic ability.

Collegiate Division 1 swimming results illustrate this. The record times for every individual event are better for men than for women by an average of 13%. Track and field events show the same disparity. For example, in the 2021 Olympics, if men and women had competed together in the final of the 200 meter run, the top 8 (of 16) places would have been taken by men. These male/female performance differences are not because men are more dedicated or train harder, it’s because, well, they are men.

Trans women usually take testosterone-suppressing treatments. But in about 75% of cases these treatments do not bring testosterone down to levels typical of genetic females (which are 5-8% of genetic male levels), and in fully one-quarter of cases there is no testosterone reduction at all. More important, even successful suppression of testosterone does not fully reverse the effects of having high testosterone levels earlier in life. Look at photos of the trans college swimmer Lia Thomas next to her competitors and note her advantages in height, shoulder width, and arm length.

Trans advocates argue that trans people should be able to compete on the team that corresponds to their gender identity. But identity does not erase biology. Consider this hypothetical situation: David is on his school’s wrestling team. He weighs 180 pounds but he identifies as weighing 150 pounds. Can he compete in the 150 pound class? I don’t think anyone would argue that this is OK.

We should all wish our trans friends and neighbors good lives free of discrimination. But it is not discrimination to take biological facts into account when organizing athletic events that are fair for all competitors.