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Fighter pilots and astronauts have really intense jobs, and they put their bodies under extreme amounts of physiological stress. Just watch five minutes of Apollo 13 or Captain Marvel, and you will know exactly what I'm talking about.

This kind of stress — especially the high gravitational forces — is so significant that some people believe it can affect their kids. More specifically, there's this common idea in flying communities that if someone who produces sperm wants to be a fighter pilot and an amazing parent, they should prepare for a household full of daughters. The idea is that a sperm carrying a Y chromosome is smaller and more fragile than one carrying an X, so it's more easily disrupted by things like high gravitational forces.

But before you go start switching careers or anything, you might want to look at the research. Typically, making a baby is pretty straightforward — at least, on a cellular level. In most cases, you start with an egg cell carrying an X chromosome.

And then that egg combines with a sperm cell carrying either an X or a Y chromosome. And ultimately, you get an XX or XY kid. Although there is plenty of nuance, most females are XX, most males are XY.

For most of the population, the odds of a sperm cell contributing an X or a Y is about 50/50. But if you look at certain studies, it seems like that might not hold true for pilots. Take a 1987 study published in the journal Aviation, Space, and Environmental Medicine.

It looked at the number of sons and daughters of 62 male tactical pilots and astronauts. Then, it compared those numbers to those of 222 men who had regular day jobs or were pilots with a lower exposure to gravitational forces. The analysis found that the tactical pilots and astronauts tended to have more daughters than the average Joes they were compared to.

Now, to be clear, the researchers just pulled this data from biographies; it's not like they ran genetic tests or anything. But it's likely that in this paper and in the others we mention, daughters were XX, and sons were XY. Still, one paper is rarely enough to prove anything.

There might be more going on here, so it's important to look at other research. There have been several other studies about this, but in 2009, an especially large one was published in the journal Military Medicine. It compared the families of more than 500 naval aviation officers to those of the general population.

And on average, it found almost no difference in the kind of kids each group had. The one significant difference it did find was among pilots who flew in extreme environments about two months before they conceived. That group was more likely to have daughters.

In human reproduction, timing is everything. So maybe this was a sign that the stress of flight was damaging Y sperm right before conception. Still, it's hard to take that answer and run with it, because scientists can't actually explain why this would happen.

Many people used to believe that Y sperm were smaller, which would make them more susceptible to damage. But that was based on research from the 1960s, and today, we know that isn't true. As far as we can tell, there is actually no meaningful size difference between X and Y sperm.

So it's hard to figure out what's going on with those pilots. There are plenty of hypotheses floating around, but at this point, it's too soon to say anything for sure. At the end of the day, though, individual pilots probably shouldn't be worried.

As that 2009 paper pointed out, even if the odds are skewed toward XX kids on a population level, that doesn't mean they are for every individual. Ultimately, if you're a pilot, you probably can't predict what kind of kids you're going to have any better than the rest of us here on the ground. So… welcome to the club. [♪ OUTRO].