Garment fit, scrotal thermoregulation, and male fertility outcomes: a narrative review

Introduction

Scrotal thermoregulation plays a critical role in normal spermatogenesis, with the human testes functioning optimally at temperatures several degrees below core body temperature. One factor of longstanding interest is scrotal temperature: the testes operate optimally at temperatures slightly below core body temperature, and even mild increases can impair spermatogenesis (1). Tight-fitting underwear has therefore been hypothesized to elevate scrotal temperature by limiting ventilation and heat dissipation, potentially exerting adverse effects on spermatogenesis. Conversely, looser-fitting undergarments, such as boxer shorts, may allow improved ventilation and heat dissipation, potentially preserving normal spermatogenesis (2,3).

In recent years, questions regarding underwear choice and fertility have also gained renewed public attention, driven in part by online health information and social media discussions surrounding male reproductive optimization. This heightened visibility has been accompanied by increased consumer interest in loose-fitting and cotton-based underwear marketed for “fertility-friendly” purposes, despite ongoing uncertainty regarding the magnitude of any clinical benefit. Against this backdrop, a careful synthesis of the scientific evidence is warranted to guide evidence-based patient counseling.

Beyond garment fit, the material composition of underwear may also influence the scrotal microenvironment. Breathable natural fibers such as cotton may facilitate cooling, whereas synthetic fabrics may retain heat and moisture; however, empirical evidence supporting these assumptions remains limited. Experimental textile data provide indirect mechanistic insight, demonstrating that cotton-containing fabrics generally exhibit higher thermal conductivity and lower thermal resistance, promoting greater heat dissipation, whereas polyester-based fabrics tend to demonstrate higher thermal resistance, consistent with increased insulation and heat retention. However, these effects are strongly influenced by fabric structure and layering rather than fiber type alone (4).

Within this context, understanding whether such material-dependent differences translate into clinically meaningful reproductive effects remains essential. This narrative review synthesizes the available human and experimental literature evaluating whether underwear choice, particularly garment fit and fabric type, has a measurable impact on semen parameters and male fertility outcomes, with an emphasis on clinically relevant evidence to inform men’s health practice. We present this article in accordance with the Narrative Review reporting checklist (available at https://tau.amegroups.com/article/view/10.21037/tau-2026-1-0139/rc).

Methods

We conducted a structured narrative literature review to evaluate the relationship between underwear choice, scrotal temperature, and male reproductive parameters, including semen quality and fertility-related outcomes. A comprehensive literature search was performed in PubMed, which was the sole database used, to identify relevant studies published through 2025.

The search strategy employed combinations of the following terms: (“underwear” OR “boxers” OR “briefs” OR “undergarments”) AND (“scrotal temperature” OR “testicular temperature”) AND (“male fertility” OR “sperm quality” OR “semen parameters” OR “sperm concentration” OR “sperm motility” OR “sperm morphology” OR “fecundity”).

Studies published up to April 20, 2025 were considered; no date restrictions were applied. Only peer-reviewed, English-language human studies were included. Reference lists of included articles were also reviewed to identify additional relevant publications. A summary of the literature search strategy and study selection process is presented in Table 1.

We included observational studies (cross-sectional, case–control, and cohort designs) and interventional studies that examined underwear type, garment fit, or fabric composition in relation to semen parameters, hormonal markers of testicular function, or fertility-related outcomes. Experimental studies involving controlled scrotal heating were also included to provide biological context for the effects of temperature on spermatogenesis. Studies were eligible if they (I) compared at least two types of underwear or exposure conditions relevant to scrotal temperature; and (II) reported outcomes such as semen volume, sperm concentration, total sperm count, motility, morphology, and, in selected studies, sperm DNA integrity, reproductive hormone levels, or measures of fecundity (e.g., time to pregnancy).

Titles and abstracts were screened for relevance, followed by full-text review of selected articles. Data extracted from each study included population characteristics (e.g., fertile men, subfertile clinic populations, couples attempting conception), study design, sample size, exposure definitions, and reproductive outcomes assessed. Extracted study characteristics were used to compare findings across studies and to identify recurring patterns in reported associations between underwear type, scrotal temperature, semen parameters, hormonal markers, and fertility-related outcomes.

Due to substantial heterogeneity in study designs, populations, exposures, and outcomes, findings were synthesized qualitatively rather than quantitatively. After data extraction, studies were grouped by major topic area, including garment fit, fabric composition, scrotal temperature effects, semen parameters, and fertility-related outcomes. Study findings were summarized descriptively, and key study characteristics and outcomes were compiled in Table 2 to support the thematic narrative synthesis.

Results

Garment fit: boxers versus briefs

The relationship between underwear tightness and male fertility has been explored sparingly for several decades, with mixed results. Representative study characteristics and quantitative findings are summarized in Table 2. Among men undergoing infertility or subfertility evaluation, early studies comparing self-reported underwear type within the same clinical population did not identify major differences in semen parameters. For example, Oldereid et al. reported similar sperm concentration (81.8 vs. 81.7 million/mL), progressive motility (20.2% vs. 22.9%), and percentage of abnormal forms (57.2% vs. 54.8%) in men wearing close-fitting versus loose-fitting underwear (5). Likewise, Munkelwitz and Gilbert did not observe statistically significant differences in sperm concentration, motility, or percentage of normal WHO forms between boxer and brief wearers (2).

In contrast, Parazzini et al. reported elevated odds of dyspermia among men who usually wore tight underpants and tight trousers, although these individual estimates did not reach statistical significance; the highest odds were observed among men reporting both tight underpants and tight trousers (6). Taken together, these findings suggest that while observational data remain inconsistent, there may be a trend toward adverse effects with tighter garments that is not consistently captured in clinical semen parameters alone.

Physiologic studies provide a plausible mechanism for these observations. Controlled measurements of scrotal temperature in Jung et al., obtained using bilateral thermistor sensors attached to the scrotum during standardized sitting and walking periods, demonstrated that tight-fitting undergarments can increase scrotal temperature by approximately 1 ℃ (3). Mieusset and Bujan likewise used an induced-heating model in which the testes were maintained close to the inguinal canal during waking hours, producing an estimated 1 to 2 ℃ increase in testicular temperature (1). These findings could support the biological premise that garment fit may influence testicular heat exposure.

Subsequent epidemiologic studies continued to yield heterogeneous results. Case-referent analyses identified an association between not wearing loose underwear and poorer semen quality, including lower motile sperm concentration (7). In a study of 344 men attending an infertility clinic, Jurewicz et al. used interview-based lifestyle data and semen testing and reported more favorable semen findings among men who predominantly wore loose-fitting underwear (8). However, Pacey et al., in a multicentre UK case-referent study including 318 cases and 1,652 referents, used self-reported underwear type and centralized morphology assessment and did not identify a meaningful association between underwear type and sperm morphology (9).

A 2016 prospective cohort study of approximately 500 men in couples attempting conception examined both daytime and nighttime underwear habits in relation to semen quality and fecundity. Although modest differences in semen parameters were observed across underwear categories, no significant differences in time-to-pregnancy were detected over follow-up (10). These findings suggest that while underwear choice may modestly influence sperm production, available data have not demonstrated meaningful differences in clinical fertility endpoints, and its impact on fertility outcomes appears limited for most couples.

More recently, however, a larger cross-sectional study provided additional evidence that looser underwear may be associated with improved testicular function. A 2018 cross-sectional study of over 650 men presenting for fertility evaluation demonstrated a consistent association between looser underwear and markers of improved testicular function. Men who predominantly wore boxer shorts had approximately 20–25% higher sperm concentration and total sperm count, along with lower serum follicle-stimulating hormone (FSH) levels, compared with men wearing tighter underwear (11). These findings suggest reduced compensatory gonadotropin stimulation and more efficient spermatogenesis among men avoiding tight garments. No differences in sperm motility, morphology, or DNA fragmentation were observed (11).

Taken together, the literature suggests that tight underwear may modestly elevate scrotal temperature and may be associated with reduced sperm production in some men. The magnitude of observed differences, often on the order of 15–25% in sperm concentration, may be clinically relevant for men near subfertile thresholds but is unlikely to affect fertility in men with otherwise normal semen parameters meaningfully. Fertility guidelines emphasize avoidance of excessive scrotal heat exposure, although they do not specifically address underwear choice (12).

Testicular heat exposure and fabric considerations

In addition to garment fit, the scrotal microenvironment, particularly temperature, is an important determinant of male reproductive physiology. Underwear fabric has been discussed as one factor that may influence this environment (13). However, direct human evidence evaluating the effects of fabric composition under typical wearing conditions is lacking. To date, no human studies have directly measured scrotal temperature or fertility outcomes comparing natural versus synthetic underwear fabrics in real-world use. As such, available data addressing fabric composition are indirect and must be interpreted cautiously.

Experimental studies of scrotal hyperthermia demonstrate that spermatogenesis is highly sensitive to small, sustained increases in testicular temperature, providing physiologic context for clothing-related exposures. A controlled trial inducing mild scrotal heating through supra-scrotal positioning of the testes resulted in marked reductions in sperm concentration and motility, with several participants developing severe oligozoospermia or azoospermia during the intervention period. Normal spermatogenesis resumed after removal of the heat stress, indicating reversibility (1). Additional studies using polyester-lined athletic supports increased scrotal temperature by approximately 0.8 to 1.0 ℃ but did not demonstrate significant declines in sperm concentration, motility, morphology, viability, hyperactivation, or sperm function over time (14). More recent work has shown that heat stress may impair sperm chromatin integrity before substantial reductions in sperm count become apparent, suggesting that thermal exposure can affect both sperm quantity and quality (15). Importantly, these studies do not evaluate fabric type, but rather establish the physiological consequences of testicular heat exposure.

One small experimental study reported reversible azoospermia among men wearing polyester underwear continuously for several months (13). In that study, suppression of spermatogenesis was not attributed to thermal effects, but instead to electrostatic potentials generated by friction between polyester fibers and the skin, a mechanism hypothesized by the authors to interfere with testicular function. This proposed mechanism is biologically unconventional and has not been independently validated. The study’s very small sample size, continuous-wear design, and lack of direct physiologic measurements substantially limit generalizability, and typical daily use of synthetic underwear is unlikely to reproduce these conditions. Accordingly, this study should be regarded as hypothesis-generating rather than evidence of clinically relevant fabric effects.

Discussion

This review synthesizes evidence suggesting that underwear choice may influence male reproductive physiology, primarily through effects on scrotal temperature. Normal spermatogenesis depends on effective scrotal thermoregulation, with the testes positioned outside the body within the scrotum to maintain a temperature several degrees below core body temperature. This cooler environment is preserved through anatomic positioning and physiologic heat dissipation across the scrotal skin (16). Under physiologic conditions, these mechanisms help maintain the testes within a thermoneutral range that supports normal spermatogenesis.

Although experimental mammalian studies have suggested limited thermotolerance with repeated heat exposure, convincing evidence for a protective adaptive response in humans remains lacking (15,16). Tight-fitting garments modestly increase testicular heat exposure, which can impair spermatogenesis and may trigger compensatory hormonal responses, as reflected by higher FSH levels in some observational studies (11). Mechanistically, heat exposure appears to interfere mainly with germ cell maturation and, to some extent, Sertoli cell function, while testosterone production by Leydig cells seems to remain largely intact (16). At the same time, heat stress increases oxidative injury and apoptosis in the testis, leading to sperm DNA damage that can precede obvious reductions in sperm count (15).

Across epidemiologic studies, associations between tight underwear and poorer semen parameters have been reported inconsistently, with effect sizes generally small and dependent on population characteristics and outcome measures (2,3,5-11).

Importantly, studies assessing fertility endpoints rather than semen parameters have not demonstrated a clear benefit of switching to looser underwear (10). This suggests that underwear choice alone is unlikely to be a dominant determinant of male fertility, particularly in men with otherwise normal reproductive function. However, small improvements in sperm production may be clinically meaningful for individuals near diagnostic thresholds for oligozoospermia.

Although a precise scrotal surface temperature threshold below which spermatogenesis is uniformly preserved has not been established, basal scrotal surface temperature in healthy men is commonly reported around 33 ℃ and is often used as a reference for defining hyperthermia (17). Experimental hyperthermia studies further suggest that relatively small sustained increases in testicular temperature are associated with impaired sperm production and chromatin integrity (1,14,15). In these models, changes in motility and sperm DNA integrity have been observed within weeks (15), whereas reductions in sperm output appear to follow thereafter over the subsequent spermatogenic cycle, which in humans spans approximately 74 days, with additional epididymal transit thereafter (1,15,18). Recovery after cessation of heat exposure has generally been reported over subsequent weeks to months (1,15), although these timelines derive primarily from induced hyperthermia rather than routine underwear use.

From a clinical standpoint, current fertility guidelines emphasize avoidance of excessive testicular heat exposure, without specifically addressing underwear choice (12). In clinical practice, avoiding tight underwear or favoring loose, breathable garments may be discussed, although such guidance is not explicitly supported by guideline recommendations. When offered, this advice should be framed as low-risk and potentially supportive, rather than as an evidence-based intervention for improving fertility outcomes.

Limitations

Most human data evaluating underwear choice and fertility derive from observational studies, which are susceptible to confounding and reverse causation. Exposure assessment relies largely on self-reported underwear habits, which may vary over time and be imprecisely defined. Scrotal temperature is rarely measured directly, and underwear type serves only as a proxy for heat exposure.

Experimental studies demonstrating thermal effects often involve small sample sizes or artificial conditions that may not reflect typical daily wear. Additionally, semen parameters are imperfect surrogates for fertility, and few studies have assessed pregnancy-related outcomes. This review is narrative in nature and did not formally pool results, reflecting substantial heterogeneity in study designs, populations, exposures, and endpoints.

Future research would benefit from well-designed prospective studies that directly measure scrotal temperature in relation to specific clothing characteristics under typical real-world conditions. Standardized exposure definitions, objective temperature monitoring, and inclusion of clinically meaningful endpoints—such as time-to-pregnancy—would help clarify whether modest differences in semen parameters translate into relevant fertility outcomes. Such studies would also help distinguish the relative contributions of garment fit, fabric composition, and other sources of testicular heat exposure within a multifactorial reproductive context.

Within the broader context of male preconception care, a growing body of evidence identifies several modifiable lifestyle and environmental factors that are more consistently associated with semen quality and male reproductive health. Tobacco smoking, alcohol use, elevated body mass index, and psychological stress have each been linked to reductions in semen quality and adverse reproductive hormonal profiles in observational studies (19,20). Diet quality, physical activity patterns, and emerging microbiome-related factors may also contribute to male reproductive health, with healthier diets and moderate exercise correlating with improved motility and concentration in some cohorts (19,21,22). Emerging recent reviews additionally highlight the potential influence of sleep quality and psychosocial work stress on semen quality and reproductive outcomes, although findings are heterogeneous and mechanisms are not fully elucidated (23,24). These associations suggest that general health behaviors, rather than specific clothing choices, remain central to male fertility counseling.

Conclusions

Direct comparative studies randomizing men to cotton versus synthetic underwear under real-world conditions are lacking, and current evidence does not demonstrate that fabric composition alone produces meaningful differences in scrotal temperature or fertility outcomes. Underwear choice remains a modest and biologically plausible factor influencing male reproductive physiology, primarily through effects on scrotal temperature, and should be framed in clinical counseling as a supportive measure to minimize excessive heat exposure, with the understanding that any expected benefit is limited.

From a clinical perspective, avoidance of excessive testicular heat exposure remains the principle emphasized in fertility guidance. Any consideration of underwear choice as part of fertility counseling, therefore, represents a supportive, extrapolative measure rather than an evidence-based intervention. Such discussions should be framed within the context of a comprehensive, multifactorial fertility evaluation, as changes in underwear alone are unlikely to meaningfully alter fertility outcomes in the absence of other modifiable or treatable factors. Further research may help clarify the magnitude and clinical relevance of these associations.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://tau.amegroups.com/article/view/10.21037/tau-2026-1-0139/rc

Peer Review File: Available at https://tau.amegroups.com/article/view/10.21037/tau-2026-1-0139/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tau.amegroups.com/article/view/10.21037/tau-2026-1-0139/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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