They sought to test the theory that briefs, being more constricting, raise scrotal temperature and have a negative effect on fertility.
"Our study provides evidence that a seemingly random lifestyle choice could have profound impacts on hormone production at the level of the testis and the brain", Jorge Chavarro, an associate professor at the Harvard T C Chan School of Public Health who led the study said in a press release.
While men who wore briefs had millions fewer sperm, that wouldn't be a problem in a man with a normal sperm count, Chavarro said.
Did you know? The kind of underwear you wear can affect your sperm quality.
But men who wore boxers had 25 percent higher sperm concentrations, and 33 percent more swimming sperm in a single ejaculate, on average, than men who wore tighter underwear. Therefore, to determine the cause of male infertility, you first analyze the amount and concentration of sperm.
Still, Eisenberg said that "if the current study can be consistently reproduced, then I think it's something we should discuss with all patients".More news: Turkey Lira crisis in pictures: Locals sell wealth to save currency
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According to a paper published on Wednesday in the journal Human Reproduction, men who frequently wear loose-fitting boxers have significantly higher sperm concentrations and total sperm count than their brief-wearing brethren.
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. This proven link between temperature and fertility is often the linchpin of pro-boxer arguments presented during debates with briefs wearers.
They provided semen and blood samples and answered a questionnaire on their choice of underwear. "The testes of men wearing tighter fitting underwear would be exposed to higher temperatures than those wearing looser fitting ones". When men have trouble conceiving, "we hope that there are other ways that we can intervene that have a little more significant science and data to back them up", she said. "But if you're exercising and wearing tighter underwear, you're probably putting your testicles at risk for heat". Hence despite having poorer quality of sperms, these men had high levels of FSH. "That's why you see seasonal variations in sperm count", he added.
And while the underwear effect is not "dramatic", Fisch said.
So if you and your partner are trying to have a baby, boxers might be a good choice.
Guys with tighter-fitting pants also had higher levels of a hormone that encourages sperm production - likely to compensate for all the seed that gets fried in their high-temperature unmentionables, researchers said.