A few weeks ago, I wrote a story on the relationship (or lack thereof) between cellphone use and brain tumors. Afterward, I received a number of emails from men who said, more or less: Forget about the brain tumors. What I really want to know is whether leaving my cellphone in my pocket is affecting my sperm.
This is hardly the only sperm-related concern, and there are plenty of rumors about what does and does not affect male fertility. For example, does wearing tight underwear reduce sperm count?
I polled some FiveThirtyEight staff members, and some of my friends, about what they’ve heard. What I learned is that men have a long list of things they worry affect their sperm count: tight underwear, yes, but also laptops, weight, exercise (either too much or not enough), stress and cycling. Some of these causes for concern seemed like they must not have much of an impact on fertility (exercise cannot be both good and bad). That led me to wonder whether any of them did.
As it turns out, some of these rumors about what affects sperm count are basically myths — or, at least, not supported by much evidence. But not all of them are, and there is a basic organizing principle around male fertility: Sperm production does not go well if the testicles are too hot.
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