After exposing men to 'wet heat' for 30 minutes on alternating days, researchers found a temporary decline in sperm production but did not look at sperm quality before and after the study.

BBC: Last Updated: Monday, 5 March 2007, 11:20 GMT

While the study took into account factors that can affect sperm quality, such as age, researchers were unable to control for socio-economic factors, smoking or weight, which have been shown to have a major impact.

BBC: French sperm count 'falls by a third'

In a previous study, Agarwal and his team found that men who used their cell phones more than four hours a day had significantly lower sperm quality than those who used their cell phones for less time.

CNN: Cell phones can affect sperm quality, researcher says

It has been known for years that in a range of animals from insects to birds the males increase the quality and quantity of the sperm they ejaculate when those sperm are likely to find themselves in competition with sperm from another male.

ECONOMIST: Sex: Twin peaks | The