The men, between the ages of 32 and 39, were surveyed about the style of underwear they had been wearing for the past three months.
In a study of 656 men, by researchers at Harvard TH Chan School of Public Health, in the U.S., boxer short wearers had a 25% higher sperm concentration than men in tight-fitting underwear.
"These results point to a relatively easy change that men can make when they and their partners are seeking to become pregnant", Lidia Mínguez-Alarcón, lead author of the study and research scientist at the Harvard Chan School, said in a statement.
Researchers also found that men who wore boxers had 14 percent lower levels of follicle-stimulating hormone (FSH).
Professor Allan Pacey, of the University of Sheffield, who was not involved in the study, said: "This confirms my long-held belief that men with poor sperm quality could potentially improve things by wearing looser underwear and keeping their testicles as cool as possible".
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It's no secret that heat in a fella's nether regions reduces his potency, and tighty-whiteys tend to run hotter - but past studies comparing different underwear styles' effects on sperm production were "inconsistent", researchers noted.
More than half (345) wore boxer shorts, and only a slightly smaller number of the group wore snugger-fitting underwear: 311.
FSH is known to play an important role in male fertility and is associated with sperm production.
Among them, 53 percent wore boxer shorts. Consistent difference was found in the semen samples of these men. It was published Wednesday in the journal Human Reproduction.
Wearing looser underpants could be a simple way for men to improve their sperm count and the hormones that control it, a U.S. study suggests. "There seems some compensation, but this compensation is not sufficient to bring them back to sperm counts of men who wear looser-fitting underwear". Higher levels in men who didn't wear boxers could be evidence of the body trying to compensate for the challenge to sperm production wrought by tight underwear, Chavarro said.