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.