While past research has looked at how temperatures can affect sperm count and quality, a new study is said to be the largest of its kind to examine the potential influence of male underwear.

The paper titled "Type of underwear worn and markers of testicular function among men attending a fertility center" was published in the journal Human Reproduction on Aug. 8.

"Underwear, its parameters, and its relationship to semen quality have been under investigation for decades," said lead author Dr. Lidia Minguez-Alarcón, a researcher at Harvard University’s Department of Environmental Health. "The distinguishing feature of our study is that we specifically evaluated reproductive hormones and how they are affected by underwear." 

The participants comprised of 656 men, aged between 18 to 56 years. Health data was sourced from Massachusetts General Hospital, where the men sought infertility treatment along with their partners between 2000 and 2017.

Each participant provided their semen samples and blood work. They also answered a questionnaire about their choice of underwear. Nearly half the group (345 men, to be exact) said that they usually wore boxers while the rest wore briefs or other restrictive styles of underwear.

When comparing men based on their underwear choices, the researchers did not find any association with testosterone. But the boxer group, on average, had a 25 percent higher sperm concentration and 17 percent higher total sperm count.

Those who wore tight-fitting underwear had 14 percent higher levels of Follicle Stimulating Hormone (FSH), a possible indication of the body compensating for lower sperm concentration.

The major implication is that couples facing issues in conceiving could improve their chances if the male partner opted for loose-fitting undergarments over briefs.

"For most men, most of the time, this doesn’t mean anything — because the majority of the time, men are not actively trying to father a child," said senior author Dr. Jorge Chavarro, associate professor of nutrition and epidemiology at Harvard T.H. Chan School of Public Health.

While the sperm counts among brief-wearing participants were relatively low, it is worth noting that they were still within the normal range.

Germaine Louis, dean of the College of Health and Human Services at George Mason University, conducted similar research in the past and found no difference in how long it took people to get pregnant in relation to male underwear choices.

"Couples are already stressed out enough when they're trying for a pregnancy," she stated. "We just don't need to introduce any other stressors."

But Chavarro insisted that merely switching out one type of underwear for another was an affordable and harmless method of intervention for couples who want to improve their chances of pregnancy. 

Among other limitations, the data was self-reported and did not measure how often participants wore their preferred choice of underwear. Pants were not considered a factor even though they can have a similar impact by being restrictive or loose-fitting.