A 2017 report published in the journal Human Reproduction Update reviewed the results of over 2000 papers on sperm counts previously published (in English) between 1981 and 2013.

This comprehensive meta-regression analysis reports a significant decline in sperm counts (as measured by SC and TSC) between 1973 and 2011, driven by a 50–60% decline among men unselected by fertility from North America, Europe, Australia and New Zealand. Because of the significant public health implications of these results, research on the causes of this continuing decline is urgently needed.“

See: Temporal trends in sperm count: a systematic review and meta-regression analysis.

As stated above, the causes for the apparent dramatic decline are unknown - theories include links to body weight, a lack of physical activity, smoking, and exposure of pregnant women to traces of endocrine disruptors found in household products and foods.

The findings themselves, however, are also disputed :

Allan Pacey, professor of andrology at the University of Sheffield, said the latest study is the best he’d ever read on the decline, he said the jury is still out on whether the trend is real.
[…]

“You need to go out to answer that question. For example you take a random sample of every 18-year-old in the UK and you test 10,000 18-year-olds every year prospectively over the course of 20-25 years,” he said, emphasising that Jensen’s study using such an approach had found no decline.

Source : The Guardian , July 2017.

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