Boxers or briefs: Underwear can affect your sperm count but that's definitely not the whole story

There exists a long held belief that boxers are better for your sperm count. The rationale is that since heat can negatively affect sperm production, looser fitting underwear results in lower scrotal temperature and therefore more sperm. But, while logical, the scientific evidence to support this theory hasn't been entirely consistent, with some papers suggesting the effect of heat on sperm production is either minimal or not significant.

A recent study has tried to settle the issue by studying 656 men who presented to the infertility clinic of the Massachusetts General Hospital and analyzing their sperm counts, hormone levels, medical history and choice of underwear. The results seem to confirm the common belief, with men who wore boxers having a higher sperm count than those who did not. The average difference in sperm concentration, 65 million/mL vs. 52 million/mL, was statistically significant. However, it's not entirely clear if this difference has any implications for a man's ability to father a child.

"What's often poorly understood by many people," says Dr. Jason Elliott, an assistant professor of obstetrics and gynecology at the University of Manitoba and physician at the Heartland Fertility & Gynecology Clinic in Winnipeg, "is that there's often a very normal and significant fluctuation of semen parameters in the same man over time, with total sperm counts fluctuating by as much as hundreds of millions of sperm per ejaculate."

There has also been some debate about what qualifies as a normal sperm concentration. The definition set out by the World Health Organization sets the lower limit of normal sperm concentration at 15 million/mL, far lower than what was observed in the study. But, as Dr. Elliott points out, this number has changed over time; it used to be 20 million/mL, and doesn't reflect a hard and fast rule with respect to fertility potential. "There are men with lower concentrations of sperm that have no fertility issues and men with much higher concentrations that may never conceive," he says.

This apparent contradiction highlights an important problem in the field of infertility. There does appear to be a decline in sperm counts over time, but it's unclear how to interpret that finding and whether it has any clinical significance. One analysis showed that sperm concentration decreased from 100 million/ml to 50 million/ml between 1970 and 2010. Yet while that may seem worrisome, it's worth pointing out that these levels are still within the "normal" range. Drawing any firm conclusions is complicated by the fact that most studies have been done in men being treated in infertility clinics and therefore may not reflect what's going on in the general population. Also, sperm concentrations can fluctuate tremendously based on the last time you had sex, which most studies cannot reliably adjust for. There's also some question about how accurate the studies from the 1970s actually were.

Dr. Elliott points out that many studies testing sperm parameters rely on very few measurements per participant, often only a single measurement, and don't accurately reflect that sperm concentrations can change from one day to the next. It's also important to remember that both sperm shape and sperm motility matter when it comes to fertility, and that both of these can be abnormal even if sperm counts are in the acceptable range.

The real hitch, though, is that it's not even clear if falling sperm counts are impacting fertility rates. Birth rates are indeed falling in North America and Western Europe, but it likely has more to do with societal changes rather than biology. After all, humans are the only animals on earth that choose to limit their own fertility. Changes in the composition of the workforce, longer stretches of education and the advent of contraception have fundamentally changed when people choose to have kids.

As it stands, it's still pretty unclear if people's ability to have children has actually changed at all in recent years. According to the CDC, the rates of infertility in the United States, for one, have been more or less stable between 2002 and 2015. So a slightly lower sperm count may not be too concerning in terms of your ability to have children — but it might actually be a reflection of poor health overall. Smoking leads to lower sperm counts. So does obesity. Alcohol, drugs, diet and a lack of exercise are all potential factors in sperm count, as well.

If men want to raise their sperm count, then switching from briefs to boxers might actually do the trick, or at least help a little. The catch is that raising your sperm count by a small degree might not actually affect your fertility in any meaningful way. The real danger is that low sperm counts reflect a general trend towards worse lifestyle habits in our society. In that case, the best piece of advice may be to quit smoking, exercise regularly, eat healthy and maintain a healthy body weight. The type of underwear you wear while you do that might be less important.

Christopher Labos is a physician who writes about medicine and health issues. He co-hosts a podcast called The Body of Evidence and tweets at @drlabos.