Depending on the sport they play, athletes may not have the same probability of giving birth to a girl or a boy
A recent study has found that, among elite athletes, their sport can influence the likelihood of giving birth to a girl rather than a boy. What hypotheses might explain this “oddity”?
François Favier, University of Montpellier; Florian Britto, Paris Cité University and Grégoire Millet, University of Lausanne
Sports scientists sometimes have some strange conversations: about a year ago, we were discussing the influence of a sport on the gender of athletes’ children. It seems odd at first glance, but there was a rumor that endurance athletes had more daughters… As we talked and drew on examples from our own circles or famous athletes, this rumor seemed to hold true. But no solid scientific research had been done on the subject, so we decided to investigate! And yes, our analysis of nearly 3,000 births confirms this idea: a top-level athlete in triathlon or cross-country skiing is less likely to have a boy than a professional in team sports or tennis.
How did we arrive at these conclusions? First, we had to find the data. To do so, we used information provided by the athletes themselves via social media, specialized magazines, newspapers, Wikipedia entries, or directly through questionnaires. This marked the beginning of a long and tedious data-collection process involving professional athletes and national team rosters. We recorded the athletes’ ages, their sports, the dates of their careers, and, of course, the birth years and genders of their children. We know that in the general population, there are between 1.03 and 1.05 boys for every girl, and this ratio is very stable across the globe and over time. We compared the results from our athletes with these values and also compared the different sports to see if we could identify factors associated with the birth of girls or boys.
Nearly 3,000 births analyzed
Results: A total of 2,995 births between 1981 and 2024 to athletes from more than 80 countries and 45 different sports were analyzed, of which just under 20% were female athletes (this ratio is primarily due to data availability, which stems in part from greater media coverage of male athletes compared to female athletes). The first finding is that among our athletes, there are 0.98 male births for every female birth—fewer than among non-athletes. We’re on the right track. To see if the sport influences the sex of the children, we ranked the sports based on the percentage of male births observed in each sport and found that there are very significant differences (ranging from 56% to 35% male births) between tennis, handball, and skiing on one hand (with many boys), and cross-country skiing/biathlon, gymnastics, or long- and middle-distance running on the other.
Looking at these results, we see that the picture becomes a bit more complicated, since gymnastics and water polo—which aren’t really considered endurance sports—also seem to influence the sex of athletes’ offspring, leading to an increase in the number of girls born. On the other hand, another pattern emerges: female athletes give birth to significantly fewer boys than male athletes (0.85 boys per girl, compared to 1.02 boys per girl among men).
To gain a clearer understanding, we group the various disciplines into four categories: endurance (cycling, cross-country skiing, etc.), power (downhill skiing, jumping and throwing events, etc.), mixed (team sports), and precision (shooting, golf, etc.). We also include the following criteria: the athlete’s gender and date of birth in relation to the athlete’s career (during or after their career). We then perform a classification tree analysis. This involves dividing the sample into distinct subgroups based on the specified criteria, if those criteria have predictive power regarding the gender of the offspring.
Based on this statistical analysis, we can conclude that the sport itself is indeed the most significant factor, with athletes who participate in endurance or precision sports having significantly more daughters and fewer sons than those in the other two categories (mixed and power sports). Furthermore, within the subgroup of athletes who practice endurance or precision sports, the athlete’s own gender is a predictor of their offspring’s gender: female athletes in this subgroup have 0.7 boys for every girl, compared to 0.91 among male athletes.
Finally, as a last point, among endurance and precision athletes, having a child during or after one’s career has a significant impact, since the ratio is only 0.58 boys to 1 girl when the child is born during the career, compared to 0.81 after the career.
Ultimately, the subgroup in which the effect of high-level athletic competition is most pronounced consists of female athletes who compete in endurance or precision sports and who have a child during their careers. Among them, the probability of having a girl or a boy is 63% versus 37%, whereas it is approximately 49% versus 51% in the global population.
What are the assumptions?
How can we explain such a difference? At this point, we can only speculate.
One possible cause could be related to the parents’ hormonal profile at the time of conception. Indeed, high levels of testosterone or estrogen are thought to favor the birth of boys, whereas high levels of progesterone or cortisol do the opposite. The testosterone-to-cortisol ratio has been proposed in sports as a marker of overtraining.
Another physiological cause could be the energy expenditure associated with physical activity. In fact, embryonic development requires more energy for male fetuses than for female fetuses.
The number of hours spent training, as well as the intensity of the workouts, may alter the body’s hormonal status and/or energy state prior to conception, which could influence the sex of athletes’ offspring.
In line with this hypothesis, a study of a sample of Chilean soccer players showed that those who trained the most had more daughters than the others. The same observation holds true for animals: pregnant female mice that run the most give birth to fewer male pups. The number of weekly training hours might also explain the low proportion of boys among gymnasts and water polo players—two sports that involve a high volume of training. However, psychosocial factors could also play a role in influencing the sex of athletes’ children. For example, a high socioeconomic status has been linked to an increase in male births in the general population.
Differences in income across sports disciplines and between men and women, as well as uncertainty regarding life after a sports career, could therefore contribute to the observed variations. The list of other factors that may influence the sex of athletes’ children is long (partner profile, possible use of certain pharmacological substances, dietary and energy balance, the country’s political situation, etc.). Further standardized studies will therefore be necessary to shed light on these observations. From a physiological perspective, it would be interesting to compare the hormonal profiles, energy expenditure, and training volumes of athletes who are parents of boys with those who are parents of girls. Further studies on the sperm quality of male athletes and the adaptation of the female athletes’ reproductive tract in response to their athletic activity would also be very interesting. Finally, it would also be relevant to measure the impact of better socioeconomic support for female athletes’ careers on the sex of their offspring.
François Favier, Professor Sports Science, University of Montpellier; Florian Britto, Researcher, Paris Cité University and Grégoire Millet, Professor of Environmental Physiology and Exercise Physiology, University of Lausanne
This article is republished from The Conversation under a Creative Commons license. Readthe original article.