They also had a total sperm count an average 17 percent higher, as well as more mobile sperm.
They sought to test the theory that briefs, being more constricting, raise scrotal temperature and have a negative effect on fertility.
A researcher at the Harvard T.H. Chan School of Public Health and the study's lead author said in a press release, "These results point to a relatively easy change that men can make when they and their partners are seeking to become pregnant".
If you wear boxer shorts, your sperm will pack more of a punch.
Those with looser-fitting underwear produced up to a third more swimming sperm than those who preferred budgie-smuggler type briefs.
Chavarro told New Scientist that, although men who wear tight underwear have a lower sperm count, they generally still have sperm concentrations that fit in the average range. FSH is a hormone that is known to play a role in male fertility.
Sperm production is affected by temperature, and needs a cooler environment than in the nearby abdomen. "We're talking in the neighborhood of about 20 million sperm difference", he said.
Men who wear tight-fitting briefs have sperm counts that suffer in comparison to men who wear boxers, according to results from the largest study to date on the controversial topic. The men, all between the ages of 32 and 39, had also completed a survey that included questions about the style of underwear they had been wearing for the previous three months. This proven link between temperature and fertility is often the linchpin of pro-boxer arguments presented during debates with briefs wearers. The rest wore tighter underwear.
Men who wore boxers also had higher percentages of motile sperm or sperm that are capable of moving through the female reproductive system and fertilizing an egg.
"I have to say, though, that had I been working on male factor infertility when my wife and I were trying for a baby, I would have probably bought a few boxers". In addition to this, the researchers noticed that the shape of the sperm in these men were much better. "That's why you see seasonal variations in sperm count", he added.
It's also important to note that the study wasn't able to consider some factors that could affect testicular temperatures, such as the type of underwear fabric, or whether the men wore tight trousers such as skinny jeans, the researchers said. Underwear choice had no effect on these other markers. The researchers were able to find a potential compensatory mechanism of the hypothalamus, which increases the secretion of the hormone gonadotropin when the body detects a decrease in sperm production due to the type of underwear worn.