A 2018 study from Harvard’s fertility clinic found that men who reported wearing boxers had 25% higher sperm concentration than men who wore other styles of underwear. The study followed 656 men. The difference was measurable, not marginal. The connection between underwear and fertility is rooted in how the testicles regulate temperature-and that connection is more direct than many men realize.
Sperm production is finicky about heat. The testicles hang outside the body because they need to be roughly 2 to 4 degrees Celsius cooler than core temperature. Even small, sustained increases in scrotal temperature can temporarily reduce sperm count, motility (how well they swim), and morphology (what shape they are). The question is whether the fit and fabric of your underwear can raise that temperature enough to matter.
The Temperature Connection
The mechanism is straightforward. Tight underwear pulls the testicles closer to the body, trapping heat. Sitting for long stretches, crossing your legs, or using a laptop on your lap all add warmth, and restrictive underwear compounds those effects. In one controlled study, men who wore tight-fitting briefs had scrotal skin temperatures roughly 1°C higher than when they wore boxer shorts, measured while sitting (Jung et al., Human Reproduction, 2005). Another study asked men to wear tight briefs for 120 days and saw a rise in scrotal temperature, plus a drop in sperm concentration and total motile count (Mieusset et al., Journal of Andrology, 2007).
Sauna-level heat makes the effect more dramatic. Finnish researchers found that men who took a sauna twice a week for three months saw their sperm counts drop and their sperm morphology worsen. The good news: those changes reversed completely within six months of stopping the sauna routine (Garolla et al., Human Reproduction, 2013). The damage from heat is real but reversible, which matters when you think about underwear, because you can change underwear tomorrow.
What the Research Says About Underwear Style
The 2018 Harvard study is the strongest observational evidence we have. Researchers enrolled men at a fertility center and asked them what kind of underwear they wore most of the day. Men who chose boxers had, on average, 25% higher sperm concentration and 17% higher total sperm count compared to men who wore other styles, like briefs or boxer briefs (Minguez-Alarcon et al., Human Reproduction, 2018). The study adjusted for factors like age, body mass index, physical activity, and smoking.
This is an association, not a randomized trial. Men who wear boxers might differ from men who wear briefs in other ways that affect sperm health. But the finding lines up with what we know about temperature and spermatogenesis, and it mirrors what researchers observed when men stopped hot baths and saunas and saw semen quality recover (Shefi et al., Fertility and Sterility, 2007). The weight of evidence points toward a simple takeaway: the looser and cooler the environment, the easier it is for the testicles to maintain their preferred temperature.
Fabric, Moisture, and Microclimate
The material your underwear is made from also influences heat and humidity, even if no large study has compared polyester briefs directly with cotton boxers in terms of fertility outcomes. Cotton is soft, but it soaks up sweat and holds it against the skin, which can leave the area damp and warm through a long day. Some synthetic fabrics wick moisture away more effectively, but others trap heat like a plastic bag. A breathable construction that lets air circulate and moisture evaporate will keep the whole microclimate closer to ambient temperature.
There’s an older hypothesis that polyester underwear might generate electrostatic fields that harm sperm, but that work has not been replicated in well-controlled trials. Most andrologists today consider heat to be the primary driver of any fabric effect. Without solid data, it’s smarter to focus on what has been measured: heat and fit.
What You Can Actually Do
If you’re trying to conceive, or just want to keep your options open, the evidence suggests a few habits that favor cooler testicular temperatures:
- Choose underwear that gives the testicles room to sit away from the body. Boxers provide the most airflow, but a relaxed-cut boxer brief made from a breathable fabric can also work if you prefer support.
- Avoid prolonged sitting with your legs crossed or with a laptop resting on your lap; both add unnecessary heat.
- If you enjoy hot baths or saunas, know that the sperm effects are temporary. Stopping for a few months allows recovery, though changing your underwear is a far smaller intervention than skipping a sauna session entirely.
This is not about throwing out every pair of briefs you own. The change in scrotal temperature from underwear is modest-likely around 1°C or less-and many men who wear briefs have perfectly healthy semen parameters. But if you’re facing unexplained fertility issues, switching to looser underwear is a low-risk, low-cost step that a doctor might suggest.
Always start with a proper evaluation. A semen analysis gives you hard numbers, which is far more useful than guessing. A urologist or reproductive endocrinologist can help you pinpoint whether heat exposure, hormone levels, genetic factors, or something else is contributing.
The research makes one thing clear: what sits against your skin for 16 hours a day isn’t neutral. Small thermal nudges, repeated daily, can shift the environment in which sperm are made. That doesn’t mean underwear dictates your fertility future, but it does mean it belongs in the conversation.
This content is for educational purposes only and is not medical advice. Oakman products are designed for physical comfort and cooling; they make no claims about fertility, sperm quality, or hormone levels. Consult a healthcare professional for personalized advice.