Boxers or briefs? Largest-ever study shows boxers may beat briefs for baby-making

Men who wear boxers have significantly greater sperm concentrations and higher total sperm counts than men who wear tighter undergarments or briefs, according to a significant new study in the Journal of Human Reproduction.

The findings indicate that tighter underwear inhibits the production of sperm -- data that can be essential for couples experiencing unexplained male infertility.

Researchers found that men who wear boxers have 25 percent higher sperm concentrations and 17 percent higher sperm counts than those who wear briefs.

Previous studies have determined that higher temperatures in briefs affect sperm, but the new study is the largest of its kind -- and the first to analyze multiple styles of underwear: boxers, jockeys, briefs and others.

“Underwear, its parameters, and its relationship to semen quality have been under investigation for decades," Dr. Lidia Minguez-Alarcón, lead author and researcher at Harvard University’s Department of Environmental Health, told ABC News.

"The distinguishing feature of our study is that we specifically evaluated reproductive hormones and how they are affected by underwear," she said.

"We confirmed that underwear matters in terms of testicular function and reproductive hormones and evaluated the compensatory mechanisms involved that counterbalance lower sperm counts."

Minguez-Alarcón said the 17-year study -- from 2000 to 2017 -- is the largest and most comprehensive to date.

The study team collected semen from 656 men and blood samples from 304 men (all between 32-39 years old) who were seeking fertility treatment at the center. In addition to greater sperm concentration and sperm count, the men who wore boxers had greater sperm motility -- their sperm was better able to move -- increasing the likelihood of successfully navigating a female’s reproductive system and fertilizing an egg.

Men who wore tight-fitting underwear had 14 percent higher levels of Follicle Stimulating Hormone (FSH) compared to men who wore boxers. FSH is an endocrine hormone released by the pituitary gland into the blood to regulate sperm production, among other things.

The higher levels of FSH in the group who wore tight underwear signified the brain’s attempt to increase the production of sperm.

New study provides "clarity"

Dr. Jorge Chavarro, Associate Professor of Nutrition, Epidemiology and Medicine at Harvard University’s School of Public Health and Medical School, and a senior author of the study, said researchers hope to provide "clarity" to an important issue for would-be parents.

“There has been a scatter of smaller studies evaluating underwear and semen quality going back to the 1980s," he said. "Although the preponderance of studies show that tighter underwear leads to lower semen production, the findings often were not consistent and did not have enough statistical power."

"Our motivation was to bring clarity – we also wanted to look at testicular function more broadly," he continued. "It is just as important to note what we found and what we did not find. Even though there is a relationship with sperm production, there is no association with testosterone.”

So what does this mean for men?

Should all men go out and buy boxers?

“No," said Chavarro.

"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," he said. "Most men have no idea what their sperm count is unless men are seeing a fertility specialist.

"Depending on the patient’s baseline sperm count itself, the 25 percent decrease in sperm count observed with tighter underwear may or may not be a significant problem for that particular patient."

"Because men oftentimes are not aware of their sperm count when trying to father a pregnancy, it is a relatively simple, inexpensive lifestyle choice they can make," Chavarro said.

Change out of those briefs sooner rather than later

Chavarro said it was important to understand that changing your style of underwear won't have an immediate effect on your sperm count.

"The spermatogenic cycle takes about 90 days to fully form a mature sperm, so anything that affects sperm production takes 3 months to have an effect," he explained.

Fertility clinics and infertility services are a booming business in the U.S. -- $3.5 billion annually in 2012 -- with a growing demand.

Infertility treatment may or may not be covered by insurance –- resulting in both a cost and an extensive time commitment that is prohibitive for many and adds to the already significant financial and emotional burdens that come with infertility.

Simple and affordable interventions highlighted in this study may be welcomed by couples in the future.

Dr. Anna Chacon is a dermatologist and part of the ABC News Medical Unit.