more (at source): The researcher said: 'Sometimes, during the fine-tuning process, high rates of infertility can be seen. That's probably the reason for the very high rates of unexplained infertility in the last decades.'

Dr Hasson, of Tel Aviv University in Israel, says women's bodies have gradually evolved extra 'defences' to force sperm to become more competitive to reach the egg at all.

Men have responded by making more of the aggressive super-sperm. Once the first sperm fertilises an egg, a woman's body throws up a further range of biochemical defences to stop all the others reaching it.

Dr Hasson said: 'To avoid the fatal consequences of polyspermy, female reproductive tracts have evolved to become formidable barriers to sperm.