The researchers also noted that the men who wore boxers had 14 percent lower levels of follicle stimulating hormone, compared with men who didn't.
As it turned out, men often wore boxers (there were more than half - 345 people), sperm concentration was 25% higher, and sperm count is up 17 percent, than men, who prefer other cowards cut.
They sought to test the theory that briefs, being more constricting, raise scrotal temperature and have a negative effect on fertility.
Prof Chavarro added: "Beyond providing additional evidence that underwear choices may impact fertility, our study provides evidence, for the first time, that a seemingly random lifestyle choice could have profound impacts on hormone production in men at both the level of the testis and the brain".
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.
Those with looser-fitting underwear produced up to a third more swimming sperm than those who preferred budgie-smuggler type briefs. "Guys who wear boxers had higher sperm concentration than men who wore more tightly fitting underwear". FSH is the hormone that stimulates the testes to produce sperm.
Temperatures of between three to four degrees Celsius is required for successful sperm production. "We're talking in the neighborhood of about 20 million sperm difference", he said.
Eisenberg said that if men are having trouble conceiving, they should see a doctor for an evaluation of risk factors for fertility problems.
The study wasn't created to prove whether or how snug underwear might affect sperm production. Options included boxers, jockeys, bikini, briefs, and other.
Men trying to get their partners pregnant may not have to throw out all their briefs, said Dr. Harry Fisch, a clinical professor of urology and reproductive medicine at the Weill Cornell Medical College in New York City.
Other factors that might affect scrotal heat - such as the type of pants worn and underwear fabric - could also affect the results.
"Potentially switching from tight to loose might help some men who are on the lower edge of sperm production".
The conviction of the scientists reduced levels of this hormone in men who wear boxers, compared to those who have a more narrow trousers, due to the fact that spermiogenesis occurs normally and is not regulated by the high concentration of hormones, which takes effect at the time of damage of the scrotum is too narrow sheets.
The study was also not a clinical trial - the gold standard in health research - but a search for statistically significant correlations, which can only point to possible causes.
There have been several studies of late that show that there is a steady decline in sperm counts.