We Looked Into Whether Tight Underwear Is a Sperm-Killer

Boxers or briefs: It’s a classic question, meant to suss out something about the respondent’s personality. But for fertility doctors, that question has quite a different connotation. Whether you’re a laid-back boxers-wearer or an uptight fan of briefs, the choice may have an influence on your sperm health. Wondering which part of the underwear aisle to do your shopping in? Thankfully, science has some answers about the likelihood that your choice is affecting your sperm, and what you can do about it.

Tighter underwear styles like briefs and boxer briefs are obviously popular choices. “We have, I would say, weak to medium evidence that tight underwear may reduce sperm count or semen quality,” says Hagai Levine, an epidemiologist and male reproduction specialist at the Braun School of Public Health and Community Medicine. While the science isn’t totally clear on the extent of this effect, it's safe to say that the choice of underwear may play a part in sperm health for some, he says.

This association was bolstered by a recent study from researchers at Harvard University’s School of Public Health that found a strong correlation between underwear preference and sperm counts. In the study, the researchers asked more than 600 men who were part of couples seeking treatment at a fertility center to self-report their underwear preferences. Those who wore boxers were found to have 25 percent more sperm—and those sperm were found to be healthier.

Both groups of men, however, had sperm counts within what’s considered a normal range. Interestingly, the non-boxer wearers had higher levels of follitropin—also known as follicle stimulating hormone—which is involved in sperm production. Although other papers have found a correlation between lower sperm counts and tight underwear, this was the first to report a relationship between one of the hormones involved in sperm production and choice of underpants. The researchers cautioned, however, that their results only presented a snapshot of the men’s overall sperm health.

Researchers have been speculating about the relationship between underwear and sperm since the ‘90s. More recently, a 2016 study looked at 501 men during a year-long period of trying to conceive. The results have not pointed to a clear divide between boxers and briefs-wearers, but that doesn’t mean you shouldn’t at least think about your choice of underwear if you’re looking to have kids soon. “Right now the weight of evidence is just sort of mixed,” says Michael Eisenberg, associate professor of urology at Stanford University. When Eisenberg talks to patients, he tells them about studies like the Harvard one, but adds, “I usually counsel that they should just go with comfort. I think it's unlikely that any one layer of clothing is going to make a huge difference.”

Still, studies like the Harvard one do “point to a relatively easy change that men can make when they and their partners are seeking to become pregnant,” as study author Lidia Mínguez-Alarcón, a research scientist at Harvard's School of Public Health, said in a press release. In other words, even if we’re not sure if it’ll help, it certainly won’t hurt. Also, given that heat is bad for sperm, Levine says, it makes sense that tighter styles of underwear—which hold the testicles near the body—wouldn’t necessarily be good for your sperm. For optimal sperm health, he recommends both underwear and pants that are on the looser side.

Testicles exist outside the body for a reason, Levine says. Most mammals—elephants and sea mammals like dugongs and whales aside—have testicles that are located in a scrotal pouch away from the heat of the body. This sometimes-risky placement exists for a very good reason, Levin says; most sperm needs to be a few degrees cooler than the body’s interior in order to be healthy. Keeping the testicles in a scrotum outside the body, as most mammals do, allows them to be cool enough.

Most studies of sperm heating have been in other mammals, Levine says. These range from domesticated animals such as bulls to traditional lab animals like rats and mice. But there’s also a body of work on human sperm, which is healthiest at four to seven degrees Fahrenheit below the 98 degrees of the average human body temperature. Increasing beyond that threshold decreases in both the count and quality of the sperm, as the Harvard researchers note.

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The DNA proteins in sperm are damaged by the heat, the researchers write, which decreases their likelihood of successfully fertilizing an egg. Studies on the connection between this and underwear, however, have generally been small in size and relied on observational data from men seeking fertility treatments, Levine says. For instance, in one small French study conducted in 2012, five men spent four months wearing specially designed underwear that held their testicles close to their body cavity for 15 hours a day. The results paralleled the more recent Harvard study—sperm count decreased, and the DNA of the remaining sperm was fragmented.

Improvements in the way we gather this data could lead to more conclusive results. The thing about current studies is that there a lot of them, but they don’t do the kind of large-scale analysis that could lead to clear answers. The authors of the Harvard study caution that their results “may not be generalizable to men from the general population,” because the underwear type of the men in the study was self-reported and that factors like the type of pants the men wore, the fabric of the underwear, and other lifestyle factors might have affected the outcome.

Eisenberg would like to see the CDC start tracking male fertility like other metrics of population health by including it in their national surveys. In past conversations with the agency, he says, “there [seemed to be] some concern about the ick factor of monitoring semen quality.” But female fertility is included in the survey, and male fertility should be too, he says. “In general, most people think of fertility as a women’s problem,” he says, and there’s not a culture of men thinking about fertility.

Plenty of things besides tight bottoms, however, are demonstrably bad for male fertility—and they’re not always obvious. Multiple studies, for instance, have linked exposure to chemicals like pthalates—commonly found in soft plastics—to decreases in sperm health. Diet and obesity can also be factors, as can physical activities like bicycle riding, using saunas and hot tubs, or even having your laptop on your lap. All have been found to impact sperm count.

Like tight underwear, however, those physical causes can be mitigated by simply not doing the activity for a while, or at least being careful how much time you spend doing it. But they’re just facets of a much larger problem. Levine and his colleagues authored a 2017 meta-analysis that compiled data on sperm counts from 244 studies between the years of 1973 and 2011, finding more than a 50 percent decline in sperm count in that time.

Although some dispute the magnitude of that statistic, Eisenberg says, he also notes it’s impossible to argue that male fertility isn’t trending downwards. That’s bad for fertility outcomes, but it also signals that there’s a broader problem than boxers and briefs. “It may be a window into later [overall] health,” Eisenberg says. He’s worked on studies that show infertile men have an increased incidence of illnesses ranging from heart disease and liver disease to cancer. “Poor male reproductive health means poor male health overall,” says Shanna Swan, a professor of environmental medicine and public health at Mount Sinai and coauthor of the 2017 study.

One of the reasons it’s so difficult to make specific recommendations for treating male infertility is that there are a lot of factors—some of which start before birth. Likewise, it goes without saying that, if you’re having fertility issues, you should talk to your doctor immediately.

Swan says that men who struggle with infertility may be doing everything right in terms of their diet and other factors. They may have been exposed to environmental contaminants such as endocrine disruptors, cigarettes, or pesticides, in the womb. It’s a factor beyond your control, but the good news is that actions you take today can still have a real impact on your fertility. While specific advice for how to treat infertility can be subjective, the general rule of thumb is that what’s healthy for your body is healthy for your sperm, Levine says. That includes the obvious: Don’t smoke, exercise often, limit drinking to excess, and watch your diet. "I like to [say] anything that’s good for your heart is good for fertility,” Eisenberg says.

Swan also recommends avoiding plastic containers and non-stick cookware—both have been linked to hormone disruption. (Underwear choice, it’s worth noting, is low on her list of priorities.) Then again, it might actually come back to underwear and other clothing—not to style or fit, but to what it’s made out of, and where it was made. Levine worries about chemical exposure from clothing itself. He wants to see broader labelling requirements that will help people understand what they’re buying with their textiles. Multiple studies have found pthalates in clothing, for instance.

Chemicals are also used in the manufacturing process. “One of the major pollutant industries is the fashion industry,” Levine says. “So while making these clothes… we are polluting the environment and indirectly may impair our health and our sperm.” If all this information leaves you wondering where to start, well, a good place is assessing your overall health and what you can do to be healthier. Talk to your doctor for some first steps if you’re uncertain.

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