Padua University researchers measured temperature variations in the testicles of men using a portable computer on their laps, the Italian news agency ANSA reported.
"Up till now there had only been empirical observations that suggested an increased testicle temperature can affect the production of sperm," researcher Carlo Foresta said, referring to known risks such as wearing tight pants, taking a sauna or suffering from a fever.
Researchers used a stick-on computer chip to measure the temperatures.
"It's quite usual to see young men holding their laptops on their laps, especially on trains," Foresta said, "and we've found that this habit can raise the temperature of the testicles by two degrees in an hour."
The Padua University also examined at-risk groups like the obese and men with variococele, a condition where veins in the testicles are enlarged.
"Infertility is associated with a rise in temperature, which in both groups was about one degree," Foresta said.