The temperature in the scrotum is two to three degrees lower than body temperature. The reason for this is that sperm like it at around 34 degrees Celsius. Men who have a low sperm count are often advised to keep their testicles cool by wearing loose underwear to improve the quality of their sperm.
Until now no study has confirmed that this actually makes a difference. But in a recent study of 656 male partners of couples seeking infertility treatment, men were asked what type of underwear they were wearing. They also provided a sperm sample. About half of the men reported that they usually wore boxer shorts and they had a 25% higher sperm concentration and a 17% higher sperm count than the men who wore other types of underwear.
The effects of frequent use of laptops and obesity on the temperature in the scrotum and sperm quality have also been investigated and both seem to increase temperature which can reduce sperm quality.
So the take-home message is: keep those testicles cool by wearing loose underwear, avoiding using your laptop on your lap, and try to drop some weight if you are overweight.
Lidia Mínguez-Alarcón, Audrey J Gaskins, Yu-Han Chiu, Carmen Messerlian, Paige L Williams, Jennifer B Ford, Irene Souter, Russ Hauser, Jorge E Chavarro; Type of underwear worn and markers of testicular function among men attending a fertility center, Human Reproduction, dey259, https://doi.org/10.1093/humrep/dey259
Yefim Sheynkin, Michael Jung, Peter Yoo, David Schulsinger, Eugene Komaroff; Increase in scrotal temperature in laptop computer users, Human Reproduction, Volume 20, Issue 2, 1 February 2005, Pages 452–455,
Andrea Garolla, Mario Torino, Paride Miola, Nicola Caretta, Damiano Pizzol, Massimo Menegazzo, Alessandro Bertoldo, Carlo Foresta; Twenty-four-hour monitoring of scrotal temperature in obese men and men with a varicocele as a mirror of spermatogenic function, Human Reproduction, Volume 30, Issue 5, 1 May 2015, Pages 1006–1013