Boxers are linked to 25% higher sperm concentration. No study links them to babies.

Men who mostly wear boxers have measurably better semen parameters than men who mostly wear tighter underwear — that finding is real, published in Human Reproduction, and larger than most people would guess. What has never been shown is that switching underwear changes anyone's chance of conceiving. A 2026 review of the whole literature concluded that prospective data have not demonstrated differences in pregnancy outcomes by underwear type, and that underwear choice alone is unlikely to meaningfully influence fertility outcomes.

The theory is not the problem

The physiology is uncontroversial. Testicular function occurs at temperatures below core body temperature, and scrotal thermoregulation is a recognised feature of normal spermatogenesis [s1]. Physiologic studies have reported higher scrotal temperatures with tighter underwear [s1]. So the mechanism by which briefs might matter is real; the question is entirely about magnitude and consequence.

What the semen study found

The most-cited human study is a cross-sectional analysis of 656 male partners of couples seeking infertility treatment at a fertility centre between 2000 and 2017 [s2]. Men had a median age of 35.5 years and a median BMI of 26.3 kg/m² [s2]. About half — 53%, or 345 men — reported usually wearing boxers [s2].

Men who primarily wore boxers had 25% higher sperm concentration (95% CI 7 to 31%), 17% higher total sperm count (95% CI 0 to 28%) and 14% lower serum FSH (95% CI −27 to −1%) than men who did not [s2]. No associations were found with the other reproductive outcomes measured, which included sperm DNA damage assessed by neutral comet assay [s2].

The FSH result is what makes the paper interesting rather than merely suggestive. Sperm concentration and total count were inversely related to serum FSH, and the differences by underwear type were attenuated after adjusting for FSH [s2] — consistent with the pituitary compensating for reduced sperm production, which is the pattern you would expect if the effect were real rather than a confound.

The limits the authors themselves list

The study's stated limitations are substantial and mostly structural. It is cross-sectional, which limits causal inference [s2]. Underwear use was self-reported on a take-home questionnaire, so misclassification is possible [s2]. The men were attending a fertility centre, so the results may not generalise to men in the general population [s2]. And residual confounding remains possible because the study had no information on other things that alter scrotal heat, such as the type of trousers worn or the fabric of the underwear [s2].

That last one matters more than it sounds. A man who wears loose boxers may also wear looser trousers, sit less, cycle less, or differ in a dozen other ways from a man in tight briefs. Nothing in a cross-sectional questionnaire separates the underwear from the lifestyle it belongs to.

Where the trail goes cold

The 2026 narrative review in Translational Andrology and Urology searched PubMed to April 2025 for human studies of underwear fit or fabric against semen parameters, reproductive hormones or fertility outcomes [s1]. Its synthesis is that observational studies have described modest differences in semen parameters between men wearing briefs and boxers, and that cross-sectional analyses have reported differences in sperm concentration and FSH by underwear type — but that prospective data have not demonstrated differences in pregnancy outcomes [s1].

On fabric, the review found the evidence limited, with controlled human studies comparing materials under normal wearing conditions simply absent [s1]. Its conclusion for clinical discussions is measured: avoiding excessive scrotal heat exposure may be considered as a general measure, while emphasising that underwear choice alone is unlikely to meaningfully influence fertility outcomes [s1].

That is the gap the "fertility-friendly underwear" market sits in. A difference in a semen measurement is not a difference in a birth.

Heat that is not about underwear

There is a separate and more recent line of evidence on scrotal heat where the exposure is environmental rather than sartorial. A prospective study of 1,220 men in Utah, nested within the Folic Acid and Zinc Supplementation Trial, examined ambient temperature during spermatogenesis against sperm epigenetic age — a measure of how far age-related DNA methylation changes in sperm run ahead of or behind chronological age [s3].

Each additional 10% increase in the proportion of time exposed to wet bulb temperatures at or above the 90th, 95th and 98th percentiles across spermatogenesis was associated with 0.063 (95% CI −0.001 to 0.128), 0.121 (95% CI 0.022 to 0.220) and 0.173 (95% CI 0.030 to 0.316) years of accelerated sperm epigenetic age respectively [s3]. Associations were strongest during meiosis, at 0.146 years (95% CI 0.028 to 0.264) for the highest threshold [s3].

Those are small numbers attached to an outcome whose clinical meaning is not established — sperm epigenetic age is a research measure, not a fertility test. But the study is prospective, the exposure is measured rather than self-reported, and it points at the same physiology from a direction that does not depend on what anyone remembers wearing.

The practical summary

Heat affects sperm production; that much is settled physiology [s1]. Underwear type is associated with measurable differences in semen parameters in men attending fertility clinics [s2]. Whether changing it changes anything a couple would notice has not been demonstrated in prospective data [s1]. Anyone being sold underwear on a fertility claim is being sold the first two findings as if they were the third.

This article is informational and is not medical advice.

Sources

• Garment fit, scrotal thermoregulation, and male fertility outcomes: a narrative review — Translational Andrology and Urology, 2026-05-28
• Type of underwear worn and markers of testicular function among men attending a fertility center — Human Reproduction, 2018-07-14
• Heat exposure during susceptible windows of spermatogenesis and sperm epigenetic age — Human Reproduction, 2026-03-24