A few weeks ago, I wrote a story on the relationship (or lack thereof) between cellphone use and brain tumors. Afterward, I received a number of emails from men who said, more or less: Forget about the brain tumors. What I really want to know is whether leaving my cellphone in my pocket is affecting my sperm.

This is hardly the only sperm-related concern, and there are plenty of rumors about what does and does not affect male fertility. For example, does wearing tight underwear reduce sperm count?

I polled some FiveThirtyEight staff members, and some of my friends, about what they’ve heard. What I learned is that men have a long list of things they worry affect their sperm count: tight underwear, yes, but also laptops, weight, exercise (either too much or not enough), stress and cycling. Some of these causes for concern seemed like they must not have much of an impact on fertility (exercise cannot be both good and bad). That led me to wonder whether any of them did.

As it turns out, some of these rumors about what affects sperm count are basically myths — or, at least, not supported by much evidence. But not all of them are, and there is a basic organizing principle around male fertility: Sperm production does not go well if the testicles are too hot.

The importance of the basic heat factor is perhaps no better conveyed than in a 2012 paper entitled “Mild induced testicular and epididymal hyperthermia alters sperm chromatin integrity in men.” The authors of this paper worked with five 25- to 35-year-old French men who had at least one child to directly test the effect of heat on sperm production and sperm quality.

The five men in the study were asked to wear specially designed underwear for 14 to 16 hours a day for four months. The underwear kept the scrotum and testicles very close to the body (much closer than you’d get with, say, standard boxer briefs) and naturally heated the testicles by a significant amount relative to their normal state.

The men provided a sperm sample at several points leading up to the study, several points during the study and several points after (when they returned to wearing normal underwear). The chart below shows the sperm count and the percent of viable sperm before, during and after the experiment.

By three to four weeks into the experiment, sperm amount and quality had declined significantly. Even more striking, by the end of the study the sperm count for the men in this sample was at virtually zero. By the last visit, only three of the five men had ANY sperm in their ejaculate.

The good news? Recovery only took a few months: Once the men switched back to cooler underwear, their sperm count and quality returned to normal.

The intervention in this experiment is extreme, and so are the effects. But the general principle seems to carry over to less extreme behaviors. Tight underwear of any type seems to affect sperm count and quality.

One other example comes from a study in the Lancet, a U.K. medical journal, from the late 1990s. It followed 12 men for a year and alternated each of them between tight and loose underwear. The underwear in this case wasn’t anything special — in the loose condition, men wore boxers; in the tight condition, boxer briefs. The researchers could then compare sperm quality for each man across the two underwear types.

During the tight underwear periods, men had sperm counts of only half as much as during the loose underwear periods. Their counts of motile sperm (a measure of quality) were only a third as much during the tight periods. The effects are, of course, much smaller than in the heat-oriented study — these men still had a reasonable amount of sperm at all times — but they’re surprisingly large given the fairly low treatment intensity.

Likely for the same heat-related reason, intensive cycling is also implicated in low sperm counts. It is worth noting, however, that these effects are most conclusively demonstrated among very serious cyclists. Consider, for example, this paper from the Clinical Journal of Sport Medicine, which focused on competitive (but nonprofessional) cyclists during training. The men in this study were cycling intensively 12 to 16 hours per week. This behavior affected their sperm count but probably shouldn’t be taken to conclude that an occasional bike ride is problematic.

The downsides of heat for male fertility extend to hot baths, saunas, electric blankets, sleeping positions, even weather: Sperm counts in temperate climates are lower in the summer than in the winter. A laptop on the lap could be an issue, although as with cycling it’s likely to depend on how often you put it there (and precisely where you place it).

As for the rest of the concerns the men I spoke to raised — exercise, weight, stress — the only one whose effect on sperm is consistently supported by the evidence is weight. A higher body-mass index does seem to lower sperm count and quality. In one observational study published in the journal Human Reproduction last year, 17 percent of men with a BMI over 35 had low sperm count, versus only 3.6 percent of those with a BMI under 25. But in this same study, and others, exercise does not appear to play a role in raising or lowering sperm count.

The evidence on stress is mixed. In a comprehensive review published in 2012, several authors conclude that stress overall is associated with lower sperm count (although these effects are small relative to, say, heat). Studies that go into more detail on the type of stress seem to conclude that major life stressors — the death of a family member, divorce — matter more than everyday job strain.

In the end, for men worried about their sperm count, by far the most consistent and evidence-based guidance is to keep the testicles cool. The very good news is that the negative impacts of heat are quickly reversible. Even if you’ve been wearing boxer-briefs and enjoying a daily dry sauna, a higher sperm count is just a few months of boxer-wearing and sauna-avoidance away.

Oh, and what about the cellphone in your pocket? This does seem to be an issue in theory — if you put semen in a dish next to a cellphone and come back a few hours later, it has decreased quality relative to semen in a dish not next to a cellphone. But in humans, the effects appear to be small. In one example, only men who used the phone while it was in their pants pocket (while, say, talking on a headset) for more than four hours a day were affected. And even for them, only one measure of sperm quality declined.