For most men, the choice between boxers and briefs makes only a modest health difference, but it is not zero. The strongest evidence concerns sperm production: looser underwear keeps the testicles slightly cooler, and cooler testicles produce sperm more efficiently. One large study found that men who wore boxers had roughly 25% higher sperm concentration than men in tighter styles. Beyond fertility, underwear tightness and fabric also affect skin health, fungal infection risk, and comfort during exercise, though the stakes in those areas are lower than the fertility conversation might suggest.
Why Scrotal Temperature Is the Central Issue
The testicles hang outside the body for a reason. Optimal sperm production requires a temperature about 2 to 6 degrees Celsius below core body temperature, which puts the ideal range around 34 to 35 °C rather than the body’s internal 37 °C.1PubMed. The effects and molecular mechanism of heat stress on spermatogenesis and the mitigation measures When researchers tested sperm cell development at 35 °C versus 37 °C in a lab setting, the cells raised at the lower temperature had significantly higher survival and development rates across multiple stages of maturation.2PubMed Central. Effect of Temperature on the Development of Stages of Spermatogenesis and the Functionality of Sertoli Cells In Vitro Even a couple of degrees matters. Higher temperatures increase cell death among developing sperm and impair the support cells that nurture them.
This is where underwear enters the picture. Tight-fitting underwear presses the testicles closer to the body, reducing the natural cooling that the scrotum provides. A study that directly measured scrotal temperature found it was significantly higher in men wearing tight undergarments compared to loose ones, and significantly higher still compared to wearing none at all.3Human Reproduction. Influence of the type of undertrousers and physical activity on scrotal temperature The same study noted that physical activity like walking lowered scrotal temperature compared to sitting, which makes sense given that movement circulates air and allows the body’s cooling mechanisms to function. So the concern with briefs is not that they are inherently dangerous, but that by holding the testicles snug against the warm body core, they partially defeat the anatomy’s built-in thermostat.
What the Sperm Quality Data Actually Shows
The most-cited study on underwear and sperm quality comes from Harvard, published in 2018, and involved 656 men who were part of couples seeking fertility treatment. Men who reported primarily wearing boxers had about 25% higher sperm concentration and 17% higher total sperm count compared to men who wore tighter styles like briefs, bikinis, or jockeys. The boxer wearers also had roughly a third more motile sperm, meaning sperm capable of swimming effectively.4Human Reproduction. Type of underwear worn and markers of testicular function among men attending a fertility center
A narrative review published in 2025 corroborated these findings, summarizing the evidence as showing roughly 20 to 25% higher sperm concentration and total count in men who avoided tight garments, along with hormonal markers suggesting their testicles were under less reproductive stress.5PubMed Central. Garment fit, scrotal thermoregulation, and male fertility outcomes: a narrative review That hormonal marker deserves a closer look. The Harvard study found that boxer wearers had about 14% lower levels of follicle-stimulating hormone (FSH), a hormone the brain releases to push the testicles to produce more sperm.4Human Reproduction. Type of underwear worn and markers of testicular function among men attending a fertility center Lower FSH in the boxer group suggests their testicles were producing sperm well enough that the brain did not need to send as strong a signal. In the tighter-underwear group, the higher FSH looked like the body was compensating for reduced output by cranking up the hormonal drive.
One interesting wrinkle from a Thai study: men who wore loose underwear day and night had higher semen volume, but men who wore tight underwear around the clock actually had higher rates of normally shaped sperm.6Chula Digital Collections. The influence of fit of underpants worn on semen quality That finding cuts against the simple “loose equals better” narrative and hints that the relationship between underwear fit and sperm quality may not be entirely linear. The study was small, and the morphology result has not been widely replicated, but it is a reminder that different semen parameters can respond differently.
Does Any of This Actually Affect Fertility?
Here is where the evidence gets less dramatic. While the sperm quality numbers favor boxers, a prospective study that followed couples who were actively trying to conceive found no significant differences in time to pregnancy, conception delay, or rates of infertility based on the man’s underwear choice.7PubMed Central. Choice of underwear and male fecundity in a preconception cohort of couples That study concluded that underwear type was associated with “few differences in semen parameters” and offered reassurance to couples attempting pregnancy.
How do you reconcile a 25% difference in sperm concentration with no measurable impact on getting pregnant? A few things likely explain the gap. Sperm concentration is a lab measurement, and the normal fertile range is wide. A man who goes from 80 million sperm per milliliter to 60 million is still well within the range associated with normal fertility. The body also has that compensatory mechanism the Harvard researchers identified: when tighter underwear slightly suppresses sperm production, the brain ramps up FSH to push the testicles harder. For most men, this compensation keeps their overall fertility in a functional range. The story changes if you are already on the lower end of sperm production due to other factors like age, varicocele, smoking, or obesity. In that case, adding heat stress from tight underwear could tip the balance. But for most men with otherwise normal reproductive function, briefs are unlikely to be the deciding factor between conceiving and not.
Researchers have also acknowledged limitations in the existing studies. The Harvard study, for instance, relied on self-reported underwear type at a single point in time and did not track whether men changed their habits. Blood samples were not always taken in the morning, which could have obscured associations with testosterone and other hormones that fluctuate throughout the day.8PubMed Central. Type of underwear worn and markers of testicular function among men attending a fertility center No association was found between underwear type and testosterone levels, sperm DNA damage, or other reproductive hormones beyond FSH.
Skin Health, Fungal Infections, and the Groin Environment
Fertility is not the only health consideration below the belt. Tight, non-breathable underwear creates a warm, moist environment that fungi love. Tinea cruris, better known as jock itch, is a fungal infection that affects men about four times more often than women, and tightly fitting clothing is a recognized risk factor alongside hot or humid weather, sweating, obesity, and diabetes.9The Journal for Nurse Practitioners. Tinea cruris in men: bothersome but treatable 10Journal of New Discovery in Microbiology. Concept of tinea cruris
The mechanism is straightforward. Tight underwear reduces airflow and traps moisture against the skin, especially during physical activity or in warm weather. Fungal organisms like dermatophytes thrive in exactly these conditions. Loose-fitting underwear allows more ventilation and wicks moisture away from skin folds. If you are prone to jock itch, switching to roomier underwear and choosing moisture-wicking fabrics are standard first-line recommendations alongside antifungal treatment.
Bacterial skin infections and general irritation follow a similar pattern. Chafing is more common in tight underwear, particularly at the leg openings and waistband, and the combination of friction, moisture, and trapped bacteria can cause folliculitis or contact dermatitis. For anyone with sensitive skin or a history of groin rashes, the ventilation advantage of looser cuts is real and practical.
Fabric Choice Can Matter as Much as Fit
The boxers-versus-briefs debate tends to focus on tightness, but fabric is an underappreciated variable. A study comparing cotton and polyester workout clothing found that synthetic fabrics selectively promoted the growth of odor-causing bacteria, particularly micrococci, which were isolated almost exclusively from polyester garments. Cotton did not show the same pattern.11PubMed Central. Microbial odor profile of polyester and cotton clothes after a fitness session That finding was about shirts, but the same microbiology applies to underwear worn in a warm, sweaty environment. Polyester and nylon underwear can become noticeably smellier faster than cotton, and the bacterial communities they harbor differ from those on natural fibers.
On the chemical side, a recent review of evidence from 2019 to 2025 flagged concerns about hazardous substances in textiles generally, including dyes, plasticizers, per- and polyfluoroalkyl substances (PFAS), and metals. The review noted that chronic exposure to chemical mixtures in clothing remains poorly understood and that current safety standards often overlook how multiple chemicals interact. Particular concern was raised about phthalates in infant clothing and PFAS in water-repellent fabrics.12PubMed. Human health risks from textile chemicals: a critical review of recent evidence (2019-2025) A separate study testing clothing samples under EU regulations found that nearly two-thirds contained at least one regulated substance, with about a quarter exceeding proposed limits for bisphenols and roughly a fifth raising concerns over phthalates and polycyclic aromatic hydrocarbons.13PubMed. Unveiling the Chemical Safety of Clothing Articles, Textiles and Footwear With Regard to the Presence of Carcinogenic, Mutagenic, or Reprotoxic Substances, Endocrine Disruptors and Skin Sensitizers Under EU Regulatory Restrictions
These are general textile concerns, not specific to underwear, but underwear sits against some of the body’s most absorbent and sensitive skin for extended periods. If chemical exposure from clothing matters anywhere, it matters here. Washing new underwear before wearing it, choosing fabrics certified under safety standards like OEKO-TEX, and favoring natural fibers when possible are all reasonable precautions. That said, the overall health risk from textile chemicals in normal adult clothing appears low based on current evidence. The concern is more about long-term cumulative exposure and is most relevant for vulnerable groups like infants.
Exercise and Athletic Considerations
During exercise, the calculus shifts in favor of snugger fits. Support and compression garments reduce bouncing, chafing, and discomfort during physical activity. A study on American football athletes found significant differences in skin temperature, sweat rate, core temperature, and perceived comfort between compression girdles and standard boxer briefs during simulated practice sessions.14AATCC Journal of Research. Analysis of Predicted Thermal Manikin Physiological Responses when Wearing Compression Gear for American College and Pro-Level Football Athletes Athletes frequently prefer compression-style underwear for the physical support it provides and its ability to reduce friction during repetitive motion.
The temperature research mentioned earlier found that walking actually lowers scrotal temperature compared to sitting, suggesting that exercise itself counteracts some of the warming effect of tight underwear.3Human Reproduction. Influence of the type of undertrousers and physical activity on scrotal temperature So wearing snug athletic underwear during a run or a gym session is probably the least concerning scenario for scrotal thermoregulation, since movement and air circulation help dissipate heat. The greater thermal risk comes from sitting in tight underwear for extended periods, like during a long workday at a desk.
A practical approach that many men settle on intuitively: supportive, snug underwear for exercise, and looser styles for the rest of the day. If you spend long hours sitting, loose underwear or at least breathable fabrics make more of a difference than if you are on your feet and moving around.
When the Choice Actually Matters Most
For most men, underwear style is a comfort preference with minor health implications. But certain situations raise the stakes. If you and your partner are trying to conceive and your semen analysis has come back borderline, switching to boxers is one of the simplest and cheapest interventions available. It will not overcome a serious underlying fertility problem, but for men whose sperm counts are in the low-normal range, reducing scrotal heat could help nudge things in the right direction. The compensatory hormone response identified in the Harvard study suggests the body is already working to offset the effects of tight underwear, and removing that extra burden frees up reproductive resources.
Men recovering from varicocele repair or other scrotal surgeries are sometimes advised about underwear choice by their urologists, though recommendations vary. Some conditions call for supportive briefs in the short term for post-surgical comfort, while long-term recovery may favor looser fits to keep temperature down.
Hot working environments add another layer. If you work in a kitchen, a foundry, or any job that keeps your body temperature elevated for hours, the added heat from tight underwear compounds an already-warm scrotal environment. The same applies to prolonged sitting in heated car seats. Each of these factors individually might be insignificant, but stacked together they can push scrotal temperature further from the optimal range.
Comfort, Confidence, and the Psychology of Underwear
Health is not purely physical. Research into men’s relationship with their underwear has found that the choice carries psychological weight. A study exploring how men engage with underwear found that men use their underwear selection as a way to build comfort and confidence throughout the day, treating it as a private expression of identity and self-care.15Critical Studies in Men’s Fashion. Undressing masculinity: An exploratory study on men’s embodied engagement with underwear The right pair can affect how you carry yourself, and physical discomfort from poorly fitting underwear can be a low-grade distraction all day.
This matters because the “healthiest” underwear is not helpful if you find it so uncomfortable that you avoid wearing it. A man who finds boxers bunch up under his pants and chooses a well-fitting brief in breathable cotton is making a perfectly reasonable trade-off. The sperm quality difference, while real, is modest enough in most men that comfort should weigh heavily in the decision. And for men who are not currently trying to conceive, the fertility consideration drops even further down the priority list, leaving skin health, comfort, and personal preference as the main factors worth optimizing.
The marketing around men’s underwear has exploded in recent years, with brands promoting everything from copper-infused antimicrobial fabrics to pouch designs that claim to keep the testicles separated for better airflow. Some of these innovations address real concerns: a pouch that reduces skin-on-skin contact in the groin can reduce moisture buildup and chafing. Others are largely cosmetic. The evidence base for most of these specific design claims is thin, and no study has compared branded pouch underwear head-to-head against standard boxers for sperm quality or skin health. What the science does support is a general preference for natural or moisture-wicking fabrics, a fit that does not press the testicles tight against the body for extended periods, and whatever style you will actually wear consistently because it feels good.