Depending on their sport, athletes are less likely to give birth to a boy or a girl.
A recent study has shown that, among top athletes, their discipline can influence the probability of giving birth to a girl rather than a boy. What could explain this "strangeness"?
Favier François, University of MontpellierFlorian Britto, Université Paris Cité and Grégoire Millet, University of Lausanne
Sports science researchers sometimes have funny conversations: about a year ago, we were discussing the influence of sports discipline on the sex of athletes' children. It may seem incongruous at first glance, but the rumor was that endurance athletes had more daughters... As we discussed the matter, using our acquaintances and famous athletes as examples, this rumor seemed to be confirmed. But no solid scientific work had been done: so we investigated! And yes, our analyses of almost 3,000 births confirm the 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 of all, we had to find the data. And to do this, we used information supplied by the athletes themselves via social networks, specialist magazine websites, newspapers, Wikipedia entries or directly by questionnaire. Then began the long and tedious task of collecting data from professional sportsmen and women, or those with national team caps. We note the age of the athletes, their sporting discipline, the dates of their career and, of course, the year of birth and sex of their offspring. We know that in the general population, there are between 1.03 and 1.05 boys to 1 girl, and this result is very stable across the world and over time. We compared the results of our athletes with these values, and also the disciplines between them, to see if we could identify criteria associated with the birth of girls or boys.
Nearly 3,000 births analyzed
The result: 2,995 births between 1981 and 2024 to athletes from over 80 countries and 45 different sporting disciplines were analyzed, including just under 20% of sportswomen (this ratio being mainly due to the availability of data resulting from greater media coverage of athletes than sportswomen). The first finding is that among our athletes, there are 0.98 male births for every 1 female birth, i.e. less than among non-athletes. We're on the right track. To see whether sport influences the sex of children, we classify sports according to the percentage of male births observed in each sport, and find that there are very significant differences (56% to 35% male births) between tennis, handball and skiing on the one hand (with lots of boys), and cross-country skiing/biathlon, gymnastics or cross-country and middle-distance running on the other.
Looking at these results, we can see that the situation becomes a little more complicated, as gymnastics and water polo, which are not really considered endurance sports, also seem to influence the sex of athletes' offspring towards an increase in female births. On the other hand, there's another factor: sportswomen give birth to significantly fewer boys than sportsmen (0.85 boys to 1 girl, compared with 1.02 to 1 for men).
To make things clearer, we group the various disciplines into four categories: endurance (cycling, cross-country skiing...), power (downhill skiing, jumps and throws...), mixed (team sports) and precision (shooting, golf...). The following criteria are added: athlete's gender and date of birth in relation to the athlete's career (during or after his or her career). We then perform a classification tree analysis. This involves separating the sample into distinct sub-groups using the specified criteria, if the latter are predictive of the sex of the offspring.
In this statistical analysis, we can conclude that it is indeed the sport discipline that weighs most heavily, with endurance and precision sports athletes generating significantly more female births and fewer male births than the other two (mixed and power). Then, within the sub-group of endurance and precision sportsmen and women, the sex of the athlete himself is a predictor of the sex of his offspring: female athletes in this sub-group give birth to 0.7 boys for every 1 girl, compared with 0.91 among male athletes.
Last but not least, among endurance and precision sportswomen, having a child during or after one's career has a major impact, since the probability is only 0.58 boys to 1 girl when the birth occurs during the career versus 0.81 after the career.
Finally, the sub-group for which the effect of high-level sport is most marked is that made up of endurance and precision sportswomen who have a child during their career. For them, the probability of having a girl or a boy is 63% vs. 37%, compared with around 49% vs. 51% for the global population.
What are the assumptions?
How can such a difference be explained? At this stage, we can only speculate.
One of the causes could be linked to the hormonal profile of the parents at the time of conception. In fact, high levels of testosterone or estrogen would favor male births, while progesterone or cortisol would not. The testosterone/cortisol ratio has been proposed as a marker of overtraining in sport.
Another physiological cause could be the energy expenditure associated with physical activity. In fact, development at the embryonic stage is more energy-intensive for male fetuses than for female fetuses.
The number of hours spent training as well as the intensity of training would modify the hormonal status and/or energy state of the organism before conception, which could influence the sex of the offspring of athletes.
In line with this hypothesis, a sample of Chilean footballers showed that those who trained the most had more daughters than the others. The same applies to animals: pregnant mice who run the most have fewer male offspring. The number of hours of training per week would also explain the low number of boys in gymnastics and polo, two sports with high training volumes. But psycho-sociological aspects could also play a part in influencing the sex of athletes' offspring. For example, a good financial situation would be associated with an increase in male births in the general population.
Differences in income between sports disciplines, between men and women, or uncertainty linked to the post-career period could therefore contribute to the variations observed. The list of other parameters likely to influence the sex of athletes' children is long (partner profile, possible use of certain pharmacological substances, food/energy balance, country's political situation, etc.). Further standardized studies will therefore be needed to elucidate these observations. From the point of view of physiological hypotheses, it would be interesting to compare the hormonal profile, energy expenditure and training volume of athletes parenting boys with those parenting girls. Further studies on the sperm quality of male athletes and the adaptation of the reproductive tract of female athletes in response to their practice would also be very interesting. Finally, it would also be relevant to measure the impact of better socio-economic management of sportswomen's careers on the sex of their offspring.
Favier François, Professor in STAPS, University of Montpellier; Florian Britto, Researcher, Université Paris Cité 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. Read theoriginal article.