Importantly, while those differences were significant, none of the men had clinically low sperm counts - so while wearing looser boxers might be better for your sperm count, going for tighter ones wouldn't necessarily make you infertile.
Men who wore boxer shorts had 25 percent higher sperm concentration than those who wore briefs, researchers found.
They sought to test the theory that briefs, being more constricting, raise scrotal temperature and have a negative effect on fertility.
Dr. Jorge Chavarro, associate professor of nutrition and epidemiology at Harvard, lead author of the study has explained that tight underwear in men such as briefs can raise the scrotal temperatures and this can contribute to a lower sperm count and infertility.
In addition, there is an assumption that the quality of sperm can affect men underwear: narrower trousers can increase the temperature of the scrotum, which prevents the release of enzymes, which are responsible for sparsogenes.
Also, the study did not measure actual pregnancy outcomes, so it was unclear whether the differences in sperm count and quality uncovered actually made a bottom-line difference in the desired outcome.
The findings went on to show that the men who wore boxer shorts had a 25pc higher sperm concentration, 17pc higher sperm count and 33pc more sperm in a single ejaculate. Corroborating data on the hormone, known as FSH, was also new.
Still, Eisenberg said that "if the current study can be consistently reproduced, then I think it's something we should discuss with all patients".
But if that isn't enough to put men off of wearing them in favour of boxers, this new study just might.
"Potentially switching from tight to loose might help some men who are on the lower edge of sperm production". "The testes of men wearing tighter fitting underwear would be exposed to higher temperatures than those wearing looser fitting ones".
According to experts, the reasons for this trend are mostly external: it can be both environmental factors (e.g., poor environment), and peculiarities of lifestyle (reduced physical activity, Smoking or obesity). If your testicles feel hot and squashed, it might be time to swap to boxers or get some jeans in a larger size.
Oi, you, in the tight trousers. "That's also why we tell men not go into Jacuzzis".
The study found underwear choice could actually be a "key regulator" of sexual development and reproduction.
Regardless, if you're trying to fall pregnant, it might be worthwhile heading out to your local Kmart and picking up some boxers for your man.