Boxers or briefs? Boxers are probably a little better for sperm, most likely because looser underwear keeps the testicles slightly cooler. The best study found men who mostly wore boxers had about 25% higher sperm concentration than men in tighter underwear. But no study has shown that switching changes your chances of a pregnancy, and the NHS calls that part uncertain.
I'll be honest, I changed my underwear along with everything else when working on my own sperm quality, without checking the evidence. When I did look, the answer was more interesting than "boxers good, briefs bad".
Why Do Testicles Need to Stay Cool? (The Whole Reason This Question Exists)
Testicles sit outside the body to stay about 2 to 4°C cooler than your core temperature, and sperm production depends on that. A 2023 review in Frontiers in Cell and Developmental Biology puts the gap at 2°C to 4°C.
That's the whole logic of the boxers or briefs debate: tighter underwear holds everything closer to the body, so in theory it runs warmer.
👉 The heat part is real. The question is whether underwear matters.
What Did the Harvard Boxers vs Briefs Study Find? (The 25% Number Everyone Quotes)
Men who mostly wore boxers had 25% higher sperm concentration, 17% higher total sperm count and 33% more moving sperm than men in tighter underwear. That's from a 2018 Harvard study in Human Reproduction of 656 men attending a US fertility centre.
| Measure | Boxers (looser) | Tighter underwear | Difference |
|---|---|---|---|
| Sperm concentration | 64.7 million/mL | 51.9 million/mL | About 25% higher with boxers |
| Total sperm count | 168 million | 138 million | 17% higher with boxers |
| Total moving (motile) sperm | 70.5 million | 50.5 million | 33% higher with boxers |
| Motility (% moving) | 49.3% | 46.4% | Not a significant difference |
| Morphology (% normal shape) | 6.64% | 6.77% | No difference |
| FSH (hormone) | 7.21 IU/L | 8.20 IU/L | 14% lower with boxers |
| Sperm DNA fragmentation | Measured, not shown in main paper | Measured, not shown in main paper | No link found |
| Chance of pregnancy | no good data | no good data | Not measured |
Source: Mínguez-Alarcón et al., Human Reproduction, 2018. 656 men at a US fertility centre; adjusted averages per sample; FSH measured in 304 of the men and DNA damage in 293. Percentages are the study's own estimates, so they differ slightly from dividing the averages. "Tighter" means jockeys, briefs, bikinis, boxer-briefs or mixed.
FSH, the hormone that tells the testicles to make sperm, was 14% lower in boxer wearers. The authors suggest the body may be pushing harder to keep sperm production going in tighter, warmer conditions, but say this needs confirming.
Two details are easy to miss. Boxer-briefs counted as "tighter" in this study, not looser. And these were men at a fertility clinic who reported their own underwear, so it shows a link, not proof that boxers caused the difference.
Do Other Boxers or Briefs Studies Agree? (Not All of Them)
A big UK study points the same way, but two smaller studies found no clear difference. That's why the honest overall grade is Weak.
- UK, 14 fertility clinics: men who wore boxer shorts were about a quarter less likely to have a low concentration of moving sperm (Povey et al., Human Reproduction, 2012, 939 cases against 1,310 men without).
- US, 97 men: scrotal temperature was 33.8°C in boxers and 33.6°C in briefs, no real difference, and no difference in semen either (Munkelwitz and Gilbert, Journal of Urology, 1998). The authors' verdict: "The hyperthermic effect of brief style underwear has been exaggerated."
- US, 501 couples trying to conceive: no difference in time to pregnancy by underwear type (Sapra et al., Andrology, 2016).
The NHS lands in the same place, in its page on the causes of infertility: "There's a link between increased temperature of the scrotum and reduced semen quality, but it's uncertain whether wearing loose-fitting underwear improves fertility." NICE guidance says the same.
👉 Switching to boxers is cheap and harmless. Just don't expect miracles.
What Else Heats Up Sperm? (Often More Than Your Pants)
Saunas and hot working conditions have more direct evidence of harming semen than underwear does. This is probably where the effort is better spent.
- Saunas: twice a week for three months caused "a strong impairment of sperm count and motility" in 10 men, fully reversed six months after stopping (Garolla et al., Human Reproduction, 2013).
- Hot tubs and hot baths: when 11 men with fertility problems stopped, 5 saw their moving sperm count at least triple (Shefi et al., International Braz J Urol, 2007).
- Laptop on your lap: an hour warmed the scrotum by up to 2.8°C, though sitting with thighs together managed 2.1°C alone. Sperm wasn't measured (Sheynkin et al., Human Reproduction, 2005).
- Hot jobs: in 17 welders, normal sperm shape fell after six weeks of radiant heat and recovered after a break (Bonde, British Journal of Industrial Medicine, 1992).
👉 Heat matters more than fabric. Underwear is just the easiest bit of heat to control.
Does Heat Damage Sperm DNA? (Heat Yes, Underwear Not Shown)
Sustained heat can raise sperm DNA damage, but the Harvard study found no link between underwear type and DNA fragmentation. In a small French experiment, warming the testicles by about 2°C for around 15 hours a day pushed DNA fragmentation up from day 20 in five volunteers, and everything was back to normal by day 73 after stopping (Ahmad et al., Fertility and Sterility, 2012).
So heat can damage sperm DNA, but we couldn't find any study showing that briefs do.
How Long Until a Change Shows Up in Your Sperm? (Think Three Months)
Allow about three months. Making a sperm cell takes roughly 64 days, a figure that goes back to Heller and Clermont in Science, 1963, plus time to reach an ejaculate. In the French heat study, sperm were back to normal by day 73.
The NHS usually repeats an abnormal sperm test around three months after the first, and a semen analysis is the only way to actually know if a change worked.
It wasn't about one pair of pants. It was about taking heat out of the whole week for long enough to count.
What Actually Helps Male Fertility? (The Unglamorous List)
Loose underwear is on the NHS list for low sperm count, alongside drinking no more than 14 units of alcohol a week, stopping smoking and losing weight if you need to.
I wrote up everything I changed over six months.
When Should You See a GP?
See a GP if you haven't conceived after a year of trying (NHS). If your partner is 36 or over, don't wait: NICE guidance says you should be offered referral to a fertility specialist at your first appointment. The NHS page still says six months.
Go sooner, regardless of how long you've been trying, if you notice:
- A lump or swelling in a testicle, a change in shape or feel, or one testicle bigger than the other. The NHS says lumps can be a sign of testicular cancer, so get it checked.
- A known cause of infertility, or a history that could affect fertility, for example testicle surgery. NICE says if either partner has one, you should be offered referral at your first appointment too.
Sudden, severe testicle pain is an emergency: the NHS says go to A&E or call 999.
Boxers or Briefs: FAQ
Do boxers increase sperm count?
Boxers are linked with higher sperm counts, but not proven to raise them. In the 2018 Harvard study, men who mostly wore boxers averaged 64.7 million sperm per mL against 51.9 million in tighter underwear, about 25% higher. It was a snapshot of men at a US fertility clinic, so it shows a link rather than cause, though the NHS still lists loose-fitting underwear in its advice for low sperm count.
Is it healthier to wear boxers or boxer briefs?
Only one study has compared them directly: 501 US couples, comparing briefs, boxer-briefs and boxers, with no differences that held up statistically and no difference in time to pregnancy. In the Harvard study, boxer-briefs counted as tighter. If you want the looser option, it's boxers.
What helps a male be more fertile?
For low sperm count, the NHS advises no more than 14 units of alcohol a week, losing weight if overweight, stopping smoking, loose-fitting underwear, avoiding workplace chemicals such as pesticides and solvents, and avoiding recreational drugs including anabolic steroids. Cutting out regular saunas and hot baths is reasonable too, since small studies show heat harms sperm reversibly.
Does heat kill sperm?
Sustained heat harms sperm production. Warming the testicles by about 2°C for roughly 15 hours a day cut moving sperm from day 20 in five volunteers, and twice-weekly saunas strongly impaired count and motility in 10 men. In both, sperm recovered once the heat stopped.
How long does it take for sperm to recover after changing underwear?
Allow about three months. Sperm production takes roughly 64 days, plus time to reach the ejaculate, and after experimental heating stopped, semen measures were back to baseline by day 73. The NHS usually repeats an abnormal semen test about three months after the first. Only one tiny study has tracked an underwear switch: two men's counts drifted down and back up gradually over each three-month spell.
The Bit That Isn't About Underwear
Swapping your underwear is the easy part, and it can feel like progress when not much else does. That's worth something: trying to conceive leaves you both looking for anything you can control. It's emotional, it takes longer than you'd like, and it's something you get through together.
Keep Reading: Helpful TTC Guides
More on sperm health:
- How to Improve Sperm Quality Naturally (Real Results After 6 Months)
- Male Fertility Screening: What to Expect From a Sperm Test
- Does Alcohol Affect Fertility? What the Research Says
