A study drawing data from France, Italy, Britain, Spain and Denmark suggests that couples have more trouble conceiving where the man spends more than three hours a day in his car.

Dr. Roger Musset (Toulouse) explained the finding in terms of temperature. Above a certain temperature sperm production ceases. Men sitting for a long time in a fixed position can generate a sufficiently increased temperature in their crutch to halt sperm production.

Where men's scrotum temperature was normally between 33°C and 34°C, the 1°C rise caused by long car journeys it was not a problem. If their scrotum was naturally warmer (e.g. 36°C), the 1°C rise will stop sperm production altogether.