The advice has been repeated for decades. If you are trying to conceive, switch to boxers. The logic is straightforward. Sperm production works best a few degrees below core body temperature, and tight underwear holds the testicles closer to the body.
A new study out of Bangkok did not find that. It found the opposite on one measure, and something more complicated on another.
What 116 Men at a Fertility Clinic Actually Wore
Researchers at the Faculty of Medicine, Chulalongkorn University recruited Thai men aged 20 to 55 who were seeking infertility treatment at the Reproductive Biology Unit of King Chulalongkorn Memorial Hospital between March and June 2024. Each completed a questionnaire covering demographics, medical history, and underwear type, and provided one semen sample analyzed under the sixth edition of the WHO laboratory manual.
The habits alone are worth noting. Among the 116 eligible participants, 93.1 percent wore tight-fitting underpants during the day, while 74.1 percent switched to boxers or nothing at night.
The findings, published in July in the Chulalongkorn Medical Journal by Harnam Kaur, Unnop Jaisamrarn, Paweena Thuwanut, Chanakarn Suebthawinkul and Jiayu Li, split in two directions.
Two Results That Point Opposite Ways
Men who wore loose-fitting underwear both day and night had higher semen volume, by an average of 1.2 milliliters.
Men who wore tight-fitting underpants both day and night had a higher percentage of normally shaped sperm, by an average of half a percentage point.
Every other measure the team examined, including sperm count, motility and DNA fragmentation, showed no significant association with underwear fit.
That second finding is the counterintuitive one, and it deserves proportion. Half a percentage point in normal sperm morphology is small. The confidence interval ran from 0.04 to 0.94, meaning the true value could sit very close to zero. The authors' own conclusion is measured: a lifestyle choice like underwear may influence semen volume and sperm morphology.
Why This Does Not Settle Boxers Versus Briefs
This was a cross-sectional study. Every man gave one sample at one moment, and his underwear habit was recorded at the same time. That design can show associations. It cannot show that underwear caused anything, and it cannot rule out that some third factor, from body weight to occupation to how much time a man spends sitting, drove both the clothing choice and the semen result.
The sample is also specific. These were men already attending a fertility clinic in one city, not a general population, and 93.1 percent of them wore the same thing during the day. When almost everyone is in one category, comparisons involving the other category rest on small numbers. The authors framed the work as filling a regional gap rather than resolving the question, noting that the effect of underwear type on semen parameters remains controversial and that data from Thai men were lacking.
The wider literature has never been tidy on this question. A Lancet letter three decades ago examined tight-fitting underwear and sperm quality. A larger analysis of men attending a fertility center found that those who mainly wore boxers had higher sperm concentration and total count than men wearing tighter styles. A population-based preconception cohort looking at underwear choice and male fecundity reached more muted conclusions, and one review asked outright whether boxer shorts are really better.
There is a plausible mechanism underneath the debate. The testes sit outside the body because sperm production is temperature sensitive, and studies measuring scrotal temperature directly have shown that the type of underwear worn changes it. Whether that temperature difference is large enough or sustained enough to matter for fertility outcomes has never been settled, and posture and clothing affect scrotal temperature too.
It is also worth being clear about what these two measures actually mean. Semen volume is the amount of fluid produced, most of it from the seminal vesicles and prostate rather than the testes, and it is influenced by hydration and how long it has been since the last ejaculation. Morphology is the proportion of sperm with a normal shape, graded against strict criteria under a microscope, and it varies between laboratories. Neither number determines on its own whether a man can father a child.
The study did include sperm DNA fragmentation, a more modern measure of sperm quality that is increasingly used in fertility clinics. Underwear fit was not associated with it.
What Actually Moves the Needle on Male Fertility
Underwear is a minor variable compared with the things fertility specialists spend most of their time on. Smoking, heavy alcohol use, obesity, certain medications, infections, varicocele, and age all have far better established links to semen quality.
None of that makes the question silly. It makes it a reasonable thing to ask about and a poor thing to fixate on. A man concerned about fertility gains more from a semen analysis and a conversation with a urologist or reproductive specialist than from changing his underwear drawer.
And for anyone reading this as license to keep wearing whatever they already own, on the strength of one cross-sectional study of 116 men, that is roughly the right conclusion.
Key Questions Answered
What did the study find? Loose-fitting underwear worn day and night was associated with higher semen volume. Tight-fitting underpants worn day and night were associated with a slightly higher percentage of normally shaped sperm.
Did tight underwear reduce sperm count? Not in this study. Sperm count, motility and DNA fragmentation showed no significant association with underwear fit.
Does this prove underwear affects fertility? No. This was a cross-sectional study, which can identify associations but cannot establish cause and effect.
How big were the differences? Small. The volume difference averaged 1.2 milliliters, and the morphology difference averaged half a percentage point, with a confidence interval extending close to zero.
Who was studied? 116 Thai men aged 20 to 55 attending a fertility clinic in Bangkok between March and June 2024.
What matters more for male fertility? Factors with far stronger evidence, including smoking, heavy alcohol use, obesity, age and medical conditions such as varicocele. Anyone concerned should speak with a urologist or fertility specialist.
