For several decades it has been reasonably standard advice for men with fertility problems to avoid tight underwear, but outside of the fact that sperm is most healthy at a body temperature of around 34° C (93° F), there has been conflicting evidence on whether the type of underwear a man wears makes a great deal of difference. A new study led by Harvard T.H. Chan School of Public Health offers one of the most comprehensive examinations to date into this long-running debate.

The researchers found that men who wear boxers do indeed have better sperm counts and healthier reproductive hormone levels than their tight underwear counterparts. But again, questions still remain over how much of a difference tight undies really make on sperm quality and overall fertility.

The new research recruited 656 men, taking both sperm and blood samples alongside a self-reported questionnaire covering underwear choice, physical activity frequency and other contributing factors, such as whether they regularly take hot baths. The results were statistically significant finding that men who primarily wear boxers displayed 25 percent higher sperm concentration and 17 percent higher total sperm count.

The research also examined the men's blood samples for levels of follicle stimulating hormone (FSH). Interestingly, men wearing tighter underwear and displaying lower sperm counts were found to have higher levels of FSH in their blood. This particular hormone is known to be produced in the brain to stimulate sperm production and the researchers suggest that increased FSH levels are a sign that the body is trying to compensate for decreased sperm counts.

"We were able to find a potential compensatory mechanism whereby decreased sperm production relating to the type of underwear signals to the hypothalamus to increase secretion of gonadotropin, a hormone that acts on the testes and that is reflected by the increased levels of FSH, to try to increase sperm production," explains Lidia Mínguez-Alarcón, one of the researchers on the project.

The study may be one of the most comprehensive to date in examining the correlation between sperm health and underwear choice, but some experts do point out that there are several limitations in the research that should be taken into account. Ashley Grossman from the Oxford Centre for Diabetes, Endocrinology and Metabolism, is clear in suggesting that, while these are significant findings, there is no evidence that these changes in sperm production or health can be explicitly related to actual fertility.

"Although the sample size was large, the differences are small but then it only takes one sperm to fertilize an egg, so these are still significant findings," says Grossman. "Of course, since so many sperm are produced per ejaculate and the results are generally within the normal range, it is difficult to know if these minor changes are significant in terms of fertility."

Sheena Lewis, from Queen's University Belfast, also reiterates that the study's overall results don't actually come up with sperm count or health issues that are below a normal range, and the study isn't clear on how tighter underwear can specifically increase scrotal heat.

"The most important take home message from this study is that neither boxer shorts, jockeys nor bikinis led to a drop in sperm counts below the normal range," says Lewis.

Regardless of the limitations of the study, many experts do agree that altering the type of underwear one wears is a mild intervention that is still probably best practice for any man trying to optimize his sperm health. Allen Pacey from the University of Sheffield, affirms this conclusion, suggesting that despite a lack of clear proof that switching underwear makes any great difference, "it is a reasonable low cost and low risk lifestyle change that men with poor sperm quality can undertake to potentially improve their semen quality."

The new study was published in the journal Human Reproduction.