Well gentlemen, it’s official: Men who wear boxer shorts have higher sperm counts than those who wear briefs, according to the largest study ever to look at the link between underwear and markers of fertility. Boxer-wearing participants also had lower levels of follicle-stimulating hormone (FSH) compared to their tighty-whities-wearing peers, which the authors say indicates a healthier, more sperm-friendly environment.
The new study was published yesterday in Human Reproduction. Although previous studies have also linked tight-fitting underwear with lower sperm counts, this is the first to include information about reproductive hormones and sperm DNA damage. It also includes more participants than previous research—656 men who, along with their partners, sought infertility treatment at Massachusetts General Hospital between 2000 and 2017.
The study participants provided semen and blood samples and answered questionnaires about the type of underwear they wore most frequently. Just over half (53%) reported that they’d mostly worn boxers over the past three months, while the remainder reported wearing either briefs, bikinis (defined as very brief briefs), boxer-briefs, jockeys (which are longer and tighter, falling just above the knee), or a mix of styles.
There were some significant differences between boxer wearers and non-boxer wearers. Those who chose boxers tended to be younger, slimmer, and more likely to take hot baths or Jacuzzis than those who opted for other styles. The first two traits would likely have a positive effect on fertility, the authors note, since sperm quality decreases with age and excess weight.
Hot baths and Jacuzzis, however, could pose potential problems since heat is known to degrade sperm quality. To account for these differences between the groups, the authors controlled for these and other factors, including physical activity, smoking, and the year the sample was taken.
When the researchers crunched the numbers, they found that men who primarily wore boxer shorts averaged 25% higher sperm concentration and 17% higher total sperm count than men who wore other types of underwear. They also had 33% more motile sperm (those that were actually swimming) in each sample, and 14% lower FSH levels.
FSH is a hormone that, in men, stimulates sperm production. Higher levels in non-boxer wearers may mean that the body is trying to compensate for testicular damage caused by “elevated scrotal temperatures caused by wearing tight underwear,” the study authors wrote in their paper. (In fact, when the researchers adjusted their results to account for the difference in FSH levels, the underwear’s effect on sperm quality was no longer significant—further suggesting that the two are directly related.)
First author Lidia Mínguez-Alarcón, PhD, MPH, a research scientist at Harvard T.H. Chan School of Public Health, says that this theory requires confirmation by further research, and that the study only shows an association between underwear style and sperm count, not a cause-and-effect relationship. The findings may also not apply to the general population, since the study looked specifically at men seeking fertility treatment.
Other factors that could also affect sperm quality include exposure to heat in the scrotal area, underwear fabric, and the type of pants worn, the researchers note. (Looking at you, dudes in skinny jeans.)