This article was first published in The Skeptical Inquirer.
Did you know that the average cisgendered male human produces approximately the same number of morphologically normal sperm per day as a hamster, despite having testicles ten times the size? Yeah, me neither. Until I started researching this article, that is. Besides being another piece of trivia to add to my arsenal of hamster knowledge (Did you know that hamsters are omnivores, even though most pet hamsters are fed vegetarian diets?), this fact highlights a problem that makes researching lifestyle contributions to male fertility difficult. Human males are really inefficient at producing sperm.
As infertility researcher Richard Sharpe writes, “The poor average quality of human semen, combined with the naturally great variation in semen quality between individuals and from ejaculate to ejaculate in the same individual, means that cross-sectional studies in men face an uphill task when attempting to establish whether or not occupation, lifestyle or other environmental exposures are able to affect sperm production or quality.”
This means that while we can detect large changes in sperm quantity or quality, more subtle alterations in sperm characteristics are distinguishable only in large studies. Unfortunately, studies of such size are expensive and complex and therefore rare. Nonetheless, we can look at overall trends in the scientific literature and draw some conclusions about what facets of your life are likely to influence your fertility.
What Will Likely Affect Male Fertility
According to a 2015 review, “It is clear that marijuana and its compounds can influence male fertility at multiple levels.” Specifically, marijuana can cause decreased sperm density and low motility.
Much like marijuana, cocaine use, even only within the past five years, is associated with low sperm counts, low motility, and high concentrations of abnormal sperm cells.
A 1994 meta-analysis found that the sperm density of smokers is on average 13 percent to 17 percent lower than that of nonsmokers, although the researchers were not able to identify a dose-dependent relationship between the number of cigarettes smoked per day and sperm density.
Smoking’s effects on sperm seem to cross generations too: “After adjustment for confounding factors, men exposed to smoking in utero had a reduction in sperm concentration of 20.1% … and a reduction in total sperm count of 24.5%.” All the more reason to encourage expecting mothers not to smoke.
Much of the research on how lifestyles can affect male fertility concerns various ways in which a person’s testicles could be warmed up. Human testicles are usually maintained at 35 ˚C (95 ˚F), about two degrees lower than body temperature, and have an impressive capacity for thermoregulation. The testes are covered by the cremaster muscle, which involuntarily contracts or relaxes to either draw the testes closer to the body for warmth or away from the body to cool.
This turns out to be quite important because an increase in scrotal temperature of just a few degrees can inhibit spermatogenesis and cause infertility. One study exposed participants’ testes to microwave radiation for thirty minutes every three weeks. The results were intratesticular temperatures of 40–42 ˚C (104–108 ˚F) and infertility. A different study had participants wear insulated jockstraps for six to fourteen weeks. It likewise found a decrease in total sperm count after three weeks of use.
In line with these findings, Procopé studied twelve men and found that exposure to a sauna for a mean total of two hours and twenty-four minutes every two weeks decreased sperm counts by a maximum of 50 percent. Mieusset et al. took scrotal temperatures from 187 men and found that “the mean scrotal temperature values of the infertile men were significantly greater than those observed in the fertile men (+ 0.4°C for the right; + 0.5°C for the left).” Furthermore, several studies have found decreased fertility in men whose occupations expose them to heat, such as bakers, welders, or furnace workers.
Luckily, these effects seem to be reversible. A 2007 study involved eleven men who used hot tubs or took hot baths for at least thirty minutes per week. Three months after instructions to stop exposing themselves to these sources of heat, five of the eleven patients had a mean increase in total motile sperm counts of 491 percent! In this particular study, it seems likely that the non-responders to heat cessation were experiencing infertility effects from smoking, as non-responders had a mean of 5.6 pack-years compared to 0.11 pack-years among responders.
The study by Procopé found that exposure to warmth caused a maximum decrease in sperm counts roughly four to five weeks after heating, and studies by Rock and Robinson found that it took ten to twelve weeks for sperm counts to return to normal after heat cessation. One of the best studies on the recovery of sperm after heat exposure was done in 1963 by Tokuyama. He found that “thirty minutes of scrotal heating to 43 to 47°C induced a fall in sperm counts some 5 to 7 weeks later in 18 normal subjects, which was followed by a rapid recovery to pre-treatment counts in 4 weeks.”
One common adage warns men to not place their laptops on their laps, lest the heat from the computer cause infertility. Turns out there may be a bit of truth to this. There’s a certain amount of heat generated simply by sitting with your legs closed. In a 2004 study, scrotal temperatures increased by 2.1 ˚C when men sat with their thighs touching for sixty minutes. This likely explains why men who sit for long periods show increased rates of infertility. Sitting with a laptop increases the temperature of the testicles even more. The same 2004 study found that laptop use added another 0.5 to 0.6 ˚C of heat to scrotums. Since most of the heat is due to thigh proximity rather than the laptop itself, a cooling pad won’t solve the problem. However, a scrotal cooling patch (called FertiliMate) could—at least hypothetically. It was investigated in a clinical trial, although no results were ever published.
If you, like me, are not trying to get pregnant, you’re probably thinking that this sounds like it could be an effective method of birth control. Well, you may be right. The aforementioned Tokuyama study found that applying heat to the testicles once every three weeks was enough to keep sperm counts depressed (once every four weeks was not often enough).
Several studies have found that both sperm quantity and quality suffer when a man is exposed to stressful events. Unfortunately, stress-related infertility likely compounds with other fertility issues due to concerns over not conceiving, creating a positive feedback loop.
A 2020 review article found that “the physical and psychological trauma of war can increase the risk of infertility in men and women” and that exposure to war may be an independent risk factor for infertility in men.
What May Affect Male Fertility
a.k.a. What Should Be Studied Further
It’s the heating of the testicles that drives the idea that tight underwear or pants can cause infertility. Studies have shown that when wearing tight underwear, testicular temperatures do increase. Depending on the type of clothing worn, and the posture adopted, the increase can be anywhere from 0.5 ˚C to 0.8 ˚C. The real question is whether this is enough heating to actually cause infertility, and sadly it’s not one we have an answer to yet. Hopefully, future studies will examine whether the small temperature changes in testes induced by tight bottoms are capable of influencing fertility. For now, if you’re really worried about it, you could always switch to boxers.
At least one study has found a dose-dependent response between male body mass index (BMI) and infertility. Other research has shown negative effects on semen quality associated with BMIs over or under the “normal” range.
Unlike most mammals, humans are not seasonal breeders. Nonetheless, studies have found differences in sperm counts when samples are taken in the winter versus the summer. A study published in 1990 compared sperm samples taken in the summer and winter from 131 men and found an average 30 percent reduction in sperm counts in summer versus winter. Similarly, a 2001 study found a 28 percent reduction in sperm count in the summer, compared to the winter. Only more research will tell us whether these results are simply due to increased heat or some vestige of previous seasonal breeding preferences.
What Is Not Likely To Affect Male Fertility
Frequent sex (including masturbation)
If you were ever told to avoid masturbation or sex lest you “run out” of sperm, I have some good news: that’s almost impossible. A human man produces about 1,500 sperm cells per second, making it nearly impossible to shoot a complete blank. One study found that “average ejaculation frequency was significantly positively correlated to the motility of the sperm.” So, at least to a certain level, frequent ejaculation could actually help with infertility. However, the same study also found that ejaculation frequency was “inversely related to the proportion of sperm with abnormal morphology and semen volume.” I suppose hoping one could just masturbate their infertility away was a bit too hopeful.
While heavy drinking is implicated in fertility issues, light, moderate, or “social” drinking does not appear to be. As one researcher put it, “It appears that moderate exposure [to alcohol], to the degree experienced by the general population, is not sufficient to account for the infertile state.”
So, if you’re trying to conceive, it may be a good idea to skip the blunt, loosen your trousers, and take a shower rather than a bath. But at least you can still safely raise a glass or two (but not too many!) to the researchers who taught us all about testes.