How Tightie Whities Kill Sperm Counts

Worried about how modern life affects human sperm, both in quantity and quality? You're not alone: Ideas have been circulating that things like underwear, laptops, cycling, and weight kill off or weaken sperm. So Five Thirty-Eight starts off by tackling the tight-underwear issue: A 2012 study looked at five French fathers, ages 25 to 35, who wore tight undies that facilitated a natural heating of their testicles. After four months, their sperm quality was way down and sperm count was nearly zero. On the bright side, however, they recovered in just a few months with looser underwear. Yet that basic find—that heated testicles are temporarily bad for sperm—seems to hold true across the board:

• A dozen men who flip-flopped between boxers and boxer briefs for a year saw their sperm count drop by half during the "tighter" periods, according to a 1996 study in the medical journal Lancet.
• Serious cyclists who put foot to pedal 12 to 16 hours a week saw their sperm counts fall, according to another study—"likely for the same heat-related reason," says Five Thirty-Eight. But casual cyclists probably don't have to worry.
• Heat from electric blankets, saunas, hot baths, sleeping positions, and weather could also affect sperm if exposure is ongoing. Laptops too, possibly, if they're perched often enough on just the right spot.
• The link between sperm and weight isn't clear-cut: One study out last year found a correlation between overweight men and low sperm count; this 2012 study found a high-fat diet was tied to lower sperm production and concentration. But then there's this 2012 study, which found being overweight didn't affect sperm quality—and neither did smoking, heavy drinking, and doing drugs.
• Among the latest sperm news: A University of Otago review of 90 studies from more than 30 countries confirms there are "consistent age-related declines" in the quantity of semen and the health of sperm. Though it didn't give an age where sperm starts degrading, the lead author noted certain "well-controlled" studies have found consistent deterioration for certain sperm traits after men turn 35 or 40.

(More underwear stories.)