A 2018 study in Human Reproduction analyzed 656 men at a fertility center. Men who mostly wore boxers had sperm concentration about 25% higher and total sperm count about 17% higher than men who wore tighter underwear, after researchers adjusted for age, body mass index, smoking, and season (Minguez-Alarcon et al., Human Reproduction, 2018). The most likely explanation is temperature. The testicles need to run a few degrees cooler than the rest of the body to produce sperm well, and loose underwear lets them hang away from body heat.
That study was observational, not a randomized trial. Men chose their own underwear. The result shows a pattern, not proof that switching from briefs to boxers will change your numbers. But the mechanism behind the pattern matches what researchers understand about heat and sperm production.
What the studies found
The 2018 Minguez-Alarcon study is the main piece of evidence in this conversation. It looked at male partners of couples seeking fertility care at Massachusetts General Hospital. The men reported whether they wore boxers, briefs, boxer briefs, bikini, or jockey shorts most often. Researchers then compared semen samples.
After adjusting for age, BMI, smoking, and season, men who wore boxers had higher sperm concentration and higher total sperm count than men who wore tighter styles.
Temperature studies point the same direction. Jung et al. (Human Reproduction, 2005) found that tight underwear and sitting both raised scrotal temperature. Mieusset et al. (Journal of Andrology, 2007) compared boxer shorts with briefs and reported that boxers kept scrotal temperature lower.
Why temperature matters for sperm
Sperm production, called spermatogenesis, is heat sensitive. The testes sit outside the body because they need to be cooler than the abdominal cavity. The scrotum has two built-in controls. The cremaster muscle raises and lowers the testicles. The dartos muscle tightens or loosens the scrotal skin. Tight underwear pulls the testicles against the groin. Loose underwear gives the scrotum room to regulate temperature.
A single hot day does not cause lasting damage. Sperm cells turn over constantly. Sauna-level heat has been shown to lower sperm count and worsen morphology temporarily, with recovery after the heat stops (Garolla et al., Human Reproduction, 2013). Shefi et al. (Fertility and Sterility, 2007) reported similar recovery after men stopped wet heat exposure. Regular heat exposure matters more than a single hot day.
Where boxer briefs fit
The 2018 study grouped boxers separately from everything else. Briefs, boxer briefs, bikini, and jockey shorts were all treated as the tighter category. The comparison is loose boxers versus all tighter options combined.
Boxer briefs sit in the middle. A loose-cut boxer brief gives the scrotum more room. A compression-style boxer brief fits almost like a brief. The label matters less than the fit and how much heat is trapped. No large head-to-head trial has isolated boxer briefs from briefs, so boxer briefs remain the least studied option.
Other heat sources that affect scrotal temperature
Underwear is one heat input. Sitting for hours with thighs together adds heat. A laptop on the lap adds direct heat. Sheynkin et al. (Human Reproduction, 2005) found that laptop use raised scrotal temperature. Long hot baths, saunas, and some occupational heat exposures do the same. Underwear choice is not the only variable that influences scrotal temperature.
If you are trying to conceive, the full heat picture matters more than the label on your waistband. A man who wears boxers but works in a hot environment may still expose his testicles to regular heat stress. A man who wears briefs but moves often and avoids hot tubs may not.
Practical changes that reduce heat exposure
Sperm production takes roughly 74 days. If you change a heat-related habit today, a semen analysis about three months later will reflect most of that period. The only way to know your sperm count is a semen analysis through a clinic or lab.
Some men who are trying to conceive choose to switch to looser underwear as a low-cost experiment. Other heat-reduction habits include:
- Taking a laptop off the lap.
- Avoiding long hot baths or sauna sessions.
- Standing up or walking instead of sitting for hours.
None of these actions guarantee a higher sperm count. They simply reduce heat exposure.
If you and your partner have been trying for 12 months without success, or 6 months if the female partner is over 35, a fertility evaluation is the standard next step. Underwear is one variable worth discussing, but it is not a substitute for a medical workup.
Limits of the evidence
No randomized controlled trial has assigned men to boxers versus briefs and followed pregnancy or live birth rates. The 2018 data is cross-sectional. Men who wear boxers may also smoke less, exercise more, or have different diets. The researchers adjusted for several confounders, but unmeasured differences could still explain part of the gap. A 2026 narrative review in Translational Andrology and Urology reached the same conclusion: the association is real, but the clinical meaning for an individual man remains open.
Sperm counts vary day to day. One sample is not destiny. If your sperm count is normal, the boxer versus brief question may not change anything for you. If your count is already low, heat is one factor among many, and it belongs in a conversation with a doctor.
The evidence supports a simple mechanism. The testicles work better below body temperature, and tight underwear can hold them closer to body heat. Boxers give the scrotum more room. If you are trying to conceive, underwear is a small lever you can pull without cost or risk. It does not replace a medical workup and it does not promise a result. But it is one of the few heat variables men can control easily.
This content is for educational purposes only and is not medical advice. Consult a healthcare professional for personalized advice.