Running and testosterone: can distance determine the sex of your children?

In the world at large, about 51 per cent of live births are male children, the upward slant from 50-50 being the result of slightly faster vaginal and intra-uterine swimming speeds by sperm cells which bear the ‘Y’ chromosome (the one which confers maleness).

However, that probability of offspring maleness – 51 per cent – can vary significantly in different groups of people. For example, artists tend to have higher numbers of male children, while blacks tend to have fewer males. Dizygotic (‘fraternal’) twins tend to be male, as do their siblings, but monozygotic (‘identical’) twins are more likely to be female. Oddly, the probability of having a male child decreases as family size increases (eg, the seventh child in a family is significantly less likely to be male, compared to the first and second). And – strangely enough – distance runners tend to have fewer male children, compared to the population at large.

Why is this true? Androgens, my friend. Androgens? Yep, pure and simple, male sex hormones often seem to be the culprits. Scientists first became aware of this unusual fact – curiously enough – when they began to study men suffering from prostatic cancer. They noticed that individuals with prostate cancer, which itself is strongly linked with high testosterone levels (don’t forget that testosterone is the KEY androgen), tended to have many more male than female children. In whites, men who develop prostate cancer have about 4 per cent more male children, compared to men without prostatic tumours; in blacks, the increase amounts to more than 12 per cent (‘The Hypothesized Hormonal Control of Human Sex Ratio at Birth – an Update,’ Journal of Theoretical Biology, vol. 143, pp. 555-564, 1990).

Conversely, men predisposed to infertility because of low testosterone concentrations, tend to father females rather than males. In fact, only about 31 per cent of their children turn out to be boys. However, when such men are given high doses of testosterone to goad their gonads, they begin fathering male children, with anywhere from 58 to 85 per cent of resulting offspring being boys.

Of course, as you are already no doubt thinking, mum plays a role in determining gender, too. Perhaps, the strongest link between mothers and offspring gender has been discovered in women who suffer from toxaemia in pregnancy. Studies have shown that as many as 79 per cent of the children of such women are boys, a far cry from the usual 51-per cent ratio. Androgens are probably not the key factor here, though: Some experts suggest that ‘fatty liver’ and the high OESTROGEN levels associated with toxaemia are responsible for the hefty male output (oestrogen is the key female sex hormone).

A variety of other factors control sexual set-up at birth. For example, only about 30 per cent of the offspring of males or females who suffer from multiple sclerosis are males. Individuals who eventually develop non-Hodgkin’s lymphoma tend to produce fewer male children (and probably not coincidentally, males who develop lymphoma tend to have lower-than-normal testosterone concen-trations). Most ectopic pregnancies involve female rather than male foetuses, but females who bleed abnormally during pregnancy tend to produce more male children.

Somewhat surprisingly, studies show that ‘top-gun’ pilots of fighter aircraft are definitely not tops when it comes to either testosterone production or the fathering of male children: research suggests that they usually have low testosterone, marginal sperm counts (perhaps because of centrifugation associated with G-forces?), and daughters for descendants.

But what role does running play? If you were a male who wanted to increase your chances of fathering a female, could you put the odds in your favour by engaging in high-mileage training? After all, such exertions should lower testosterone levels, which in turn should banish males from your spouse’s belly.

Sceptics doubt that such a simple strategy would work. As they point out, humans have been doing crazy things to determine offspring gender for centuries. French noblemen of the 18th century pinched their left testicle during ejaculation, believing that the practice helped produce male heirs. At other times in human history, women wore men’s clothing while having sex, men wore boots during coitus, couples copulated at midnight in the open air, and clothing was hung on a particular side of the bed, all in hopes of engendering a male. In Austria, midwives actually buried the placenta under a nut tree – to ‘ensure’ that the next child would have a penis.

To see if running mileage might have a stronger effect than placental burial, Eddie Crawford of the University of Glasgow took a careful look at the effects of weekly mileage, training intensity, paternal age, occupation, and competitive performance on offspring gender in 139 male runners (‘Sex Ratio of the Children of Male Distance Runners,’ Thesis, Institute of Physiology, University of Glasgow, 1992). Crawford became interested in the topic when a survey appearing in the publication Scotland’s Runner revealed that just 23 per cent of the children fathered by runners engaged in serious training were boys.

Crawford divided his athletes into several categories, including (1) individuals who were not actually training at the time they and their mates conceived a child, (2) subjects who were running between 0 and 30 miles per week when their partners became pregnant, and (3) runners who were training between 30 to 50 miles per week. To these runners, a total of 377 children were born (about 3.4 per man).

Not running at all – or running less than 30 miles per week – proved to be a good way to increase one’s chances of having a male offspring. Overall, about 62 per cent of the children of runners who were taking a break from training or who were running fewer than 30 weekly miles were male.

However, things changed drastically when weekly mileage soared above 30 miles per week. For those runners who ran between 30 and 50 weekly miles, only 40 per cent of the offspring were gentlemen! According to Crawford, such high-volume running tends to produce dips in testosterone, which in turn produces a decline in the output of boys.

Mileage proved to be the only factor which had an impact on offspring gender, as running intensity (average training speed), overall life stress, paternal age, social status, manual vs. non-manual labour, competitive success, and the time interval between marriage and conception all had no significant effect.

Yet to be reckoned in the sexual-determination story is the role played by the training mileage of the mother. In theory, higher mileage levels should drive female sex-hormone concentrations downward, increasing the chances of having a female. Thus, married couples training together for a marathon and also trying to have a child should have inordinate numbers of female offspring. Conversely, such couples ambling for just 15 to 20 miles per week can look forward with some confidence to boys.

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