Bad news for guys who might like to wear tighty whities: those myths about tight underwear possibly reducing your sperm count aren't actually myths; they're very true. Sperm production just doesn't work if the testicles are too hot. It's not just your underwear that could be affecting it though, as the effect of heat on male fertility can extend to hot baths, saunas, electric blankets, sleeping positions, even using your laptop on your lap. Wow.
A study done in 2012 worked with a group of 25- to 35-year-old French men to directly test the effect of heat on sperm production and sperm quality. The men were asked to wear special underwear for 14 to 16 hours a day for four months. The underwear were, well, really really tight, and kept the scrotum and testicles very close to the body causing their boys to naturally heat up.
The men provided a sperm sample leading up to, throughout, and after the study, at which point they returned to wearing their normal undies. The chart below shows the sperm count and the percent of viable sperm before, during, and after the experiment. As you can see, by the third or fourth week, sperm amount and quality had declined significantly. Even more shocking, by the end of the study the sperm count for the men in this sample was at virtually zero. ZERO. Not one single sperm to be found.
It's worth noting that the nature of this experiment was extreme, and so are the effects. But the point is the general proof is there, that tight underwear affects sperm count and quality. The good news in all this is that recovery is possible, and only takes a few months: Once the men in the study switched back to looser, cooler, underwear, their sperm count and quality returned to normal.
So, take all those tighty whities and burn them. What are you doing wearing tighty whities, anyway?