|Sperm count measures the number of sperm present in every millilitre of semen|
Scientists found that between 1973 and 2011 there was a 59.3% fall in the average amount of sperm produced by men in Europe, North America, Australia and New Zealand.
The study is an "urgent wake-up call" for health authorities to investigate the causes and prevent it from getting worse, said Dr Hagai Levine from the Hebrew University of Jerusalem, who worked on the research.
The findings, published in the journal Human Reproduction Update, have been described by some fertility experts as "shocking".
There has been a warning of a "double whammy" caused by more women waiting until their 30s to have children.
But despite the decline, average sperm counts remained in the "normal" range, said Allan Pacey, professor of andrology at the University of Sheffield.
Sperm count measures the number of sperm present in every millilitre of semen.
A normal range is between 15 million to more than 200 million - a sperm count of less than 15 million is considered to be low.
Having a low sperm count can make it difficult to conceive a baby naturally, as well as other problems such as poor sperm quality.
Factors such as smoking, drinking, environmental factors and stress can all affect fertility.