When I’m confronted with a statement having start and end dates, I wonder, “How and why were these dates chosen?” This is an excellent question when it comes to testosterone and athletic performance. Time frames matter.

We’ve often heard data with date parameters, e.g., “Investment X has outperformed Investment Y by 2:1 over the last decade.” Often, Investment X has underperformed Investment Y by a lot over the last two decades, which goes unmentioned for obvious reasons.

Knowing that, it’s worth looking at testosterone and sports. (Disclaimer: None of the following concerns my conclusions regarding the trans conversation. I keep those to myself.)

Fact: “Boys tend to begin and end puberty sometime between age 10 and 18.”

Most readily available information has to do with testosterone and puberty. I understand that prominent visible differences manifest during puberty, e.g., muscle mass and facial hair. (This assumes that the development of the penis and testes noted at birth is not considered a prominent, visible difference, which many today do not consider to be characteristic of one sex only.)

Focusing solely on visible changes beyond what’s in a person’s underwear makes it easy to concentrate on puberty as the only important time for determining whether testosterone affects athletic performance. However, is this target too narrow?

Image: A big boy baby by freepik.

Testosterone makes its appearance before birth, i.e., in utero. It is during this period that testosterone’s influences are basically invisible (except, e.g., through ultrasound imaging, where genital development can be seen). Yet, they’re still potentially quite significant in relation to sport.

Assuming a +/- in the data (the linked article, which includes the charts reproduced below, did not include standard deviations), testosterone levels in utero are as high or higher than they are during puberty:

In the male fetus, testosterone secretion (measured by RIA) peaks in the first trimester of pregnancy between the 11th and 14th gestational week. Fetal testosterone also influences sexually dimorphic brain development. (Links to citations removed.)

If you go to the linked article and look at Figure 1, you can see clearly how testosterone rises during fetal development in male babies. Also, note in the same image that the child will have a second peak in testosterone between one and six months. Some investigators call this a “minipuberty.”

Minipuberty is the second physiological transient activation of the HPG [hypothalamic-pituitary-gonadal] axis occurring in healthy infants between ages one and six months. It plays an important role in developing genital organs and fertility in males and seems to influence the development of their body composition.

If you look at Figure 2 in the same essay, you'll see that there is no similar testosterone influence in what used to be universally known as female fetuses.

Not to be overlooked, when the fetus is exposed to testosterone in the womb, it has a “sexually dimorphic brain development” influence, i.e., it results in brain differences between what used to be uncontestedly called males and females.

These developmental brain differences have been demonstrated using multiple neuroimaging techniques:

In this paper, we reviewed evidence on sex differences in neural structure, function, metabolism and chemistry of the human brain. Evidence for sex differences stems from anatomical studies examining gray and white matter, functional neural connectivity analyses, PET studies measuring differences in neurochemistry and fMRI studies examining differences in neural activation during tasks. Overall, the various methods highlight robust sex-specific differences in human neural anatomy and function…

There are data suggesting these developmental brain variations may result in differences in cognitive development and behavior which may affect emotional expression. Dr. Pankaj Patel, from the Villanova School of Business, studied 2omb testosterone levels and their effects on adults:

Testosterone levels can be affected by social, economic, and biological factors in an individual’s life. To eliminate such variables, Patel studied twins in utero. In particular, he compared opposite-sex and same-sex twins…

Based on Patel’s research of twins, males who had higher levels of testosterone available to them in the womb are more likely to go on to be self-employed.

They are also more likely to play full contact sports like rugby or football and to work in financial services careers. In general, more testosterone is associated with lower levels of risk aversion, so people with more testosterone may take more risks. (Emphasis added.)

Note: The meanings of “male” and “female” in this paper are the “classical” ones, i.e., pre-trans. (I have tried to review his paper and have not been able to find it online. A request to him for a copy has not yet been fulfilled.)

Any or all of these testosterone-mediated differences—mental and physical—can affect differences in sports performance.

The full story is that testosterone isn’t just for puberty. Testosterone differences between people with XX and XY chromosomes start in the womb and continue during the first half-year of life. These differences are associated with cognitive, emotional, behavioral, and body composition dissimilarities that may affect how each plays sport in later life.

Perhaps a more complete understanding of testosterone’s influence from the womb will help resolve some points of contention in the Trans conversation.

Michael Applebaum, MD, JD, FCLM, is an attorney and physician in Chicago.