As a urologist at Houston Methodist, Dr. Nathan Starke gets asked one question a lot: Does my underwear choice affect my sperm? A new study suggests it can be answered by another age-old question: boxers or briefs? Though until now, he’s encountered “very little evidence” that tighty-whities actually lower sperm count, Starke tells Inverse that a new paper, showing a relationship between underwear choice and sperm concentration, is the best he’s seen yet.

In Human Reproduction, researchers from the Harvard T.H. Chan School of Public Health and Massachusetts General Hospital present evidence that men who wear boxer shorts have higher sperm concentrations than men who wear tighter-fitting underwear. While scientists have been studying the effect of underwear on sperm production for several years, this is the first attempt to investigate whether wearing boxers or briefs also affects other aspects of testicular function, reproductive hormones, and DNA fragmentation.

“We found that men who wear tighter underwear have lower sperm counts, which was consistent with previous literature,” explains co-author and research scientist Lidia Mínguez-Alarcón, Ph.D., to Inverse. “We are the first to find that those men also have higher FSH levels, suggesting a potential compensatory mechanism reflected by high FSH levels among men wearing higher underwear, compared to those who wear looser boxers.” FSH, or follicle-stimulating hormone, is known to stimulate sperm production.

Justin Bieber, briefs
Justin Bieber is a briefs man.

Approximately 53 percent of the 656 men evaluated in this study reported that they usually wear boxer shorts. These men were between the ages of 18 and 56 and were all the male partners of couples seeking infertility treatment. According to the World Health Organization reference standards, they all had “good semen quality” — that is, they had sperm that swims along in a straight line and is a normal shape, having an oval head about 5 micrometers long and 2.4 micrometers wide.

The scientist found that the men who mostly wore boxer shorts had 25 percent higher sperm count, 17 percent higher total sperm count, 33 percent more swimming sperm in a single ejaculate, and 14 percent lower FSH levels than men who typically wear tighter underpants. They also found that, although sperm DNA fragmentation couldn’t be correlated with a type of underwear, the consistently high FSH levels in men who wear tight underwear strongly suggested that they had poorer sperm quality.

“This is an intriguing observation as it implies that the wearing of tight underwear may be damaging the testicles in some way, and is why the FSH levels from the pituitary rise to try to make the testicles work harder at producing sperm,” explains Allan Pacey, Ph.D. an expert in sperm science at the University of Sheffield. Pacey was not a part of this paper, but has also found in his own research that men who wear boxer shorts are less likely to have low motile sperm counts. He’s also found a connection between wearing skinny jeans and low sperm count (sorry, emo boys of 2006).

sperm
Here are some sperm.

Mínguez-Alarcón and her colleagues think that there’s a link between underwear type and an increase in the secretion of gonadotropins like FSH. Gonadotropins are the family of gonad-stimulating hormones secreted by the hypothalamus. Increased FSH, they reason, is the result of an effort made by the body to increase sperm production. But more research will be needed to know exactly what’s happening.

“This study confirms my long-held belief that men with poor sperm quality could potentially improve things by wearing looser underwear and keeping their testicles as cool as possible,” says Pacey. “However, we should recognize that this study was not a randomized control trial. Therefore, there is no actual proof that switching underwear style will make any difference.”

It’s also important to note that while the team determined an association between underwear, semen quality, and FSH levels, the link isn’t causal, meaning no one type of underwear should be maligned just yet. Other factors might be at play as well, like the effects of scrotal heat and underwear fabric. Additionally, since the participants were all patients in a fertility study, there was no measurement of the effect of underwear on infertility here.

“The problem with cross sectional studies, like this one, is that they cannot establish a cause and effect relationship, but only a correlational one,” says Starke. “If patients are coming to see me for true infertility, there is almost invariably some other issue that has a much more dramatic impact on fertility than choice of underwear.”

But as Pacey points out that, if you’re a man who is concerned about his sperm, it’s a pretty low-effort lifestyle change to switch from briefs to boxers. You might not be able to squish them into your skinny jeans, but hey, priorities are priorities.