A large-scale meta-analysis of 185 studies across 40 years has found a more than 50 percent decline in sperm concentration and total sperm count among men from North America, Europe, Australia and New Zealand. The rate of decline was found to be consistent and is cause for significant concern if the trend continues.

Previous studies showing downward trends in global sperm counts have been criticized by scientists for a variety of factors. It has been argued that many prior fertility studies contained data that was skewed by selection bias and weren't representative of the broader population. Other researchers have also questioned the validity of historical sperm count estimates, claiming older studies were unreliable due to inaccurate measuring techniques.

This latest study attempts to correct for these limitations. Unlike earlier meta-analyses that included sample counts going as far back as 1931, this study only included data collected since 1973 so as to reduce the uncertainty in its results. Meta-regression models were also developed to account for variable factors such as age, abstinence and selection of the study population.

The results were ultimately collated from 42,935 men split into two geographic groups: Western (including North America, Europe, Australia and New Zealand) and Other (including South America, Asia and Africa).

A frighteningly steep decline was seen in both total sperm count and sperm concentration across men in Western countries. A similar trend was not seen in other geographic groups, but the researchers note that there was limited statistical data to be gathered from non-Western countries.

"Given the importance of sperm counts for male fertility and human health, this study is an urgent wake-up call for researchers and health authorities around the world to investigate the causes of the sharp ongoing drop in sperm count, with the goal of prevention," says lead author on the study Dr. Hagai Levine.

The study did not look at what could be causing this steep decline, but the researchers suspect that, "endocrine disruption from chemical exposures or maternal smoking during critical windows of male reproductive development may play a role in prenatal life, while life-style changes and exposure to pesticides may play a role in adult life."

Despite the frightening implications of this study it is important to understand the limitations of the data. While the sperm count downward trend is undeniable, it is not necessarily a clear indication of fertility as the study didn't take into account motility of the sperm.

The overall numbers also don't indicate significant increasing infertility in Western men. The study tracked a 53.4 percent drop in Western male sperm concentration, down from 99 million per milliliter in 1973 to 47 million per milliliter in 2011. Current World Health Organization standards state that any volume over 15 million sperm per milliliter is considered normal.

So while hyperbolic "human extinction" claims are swirling, we are not yet heading toward some dystopian fertility crisis à la The Handmaid's Tale or Children Of Men. Rather, this may be a sign of a greater environmental impact on male health. As the researchers note in the conclusion of the study, let this serve as a "canary in the coal mine" that signals broader issues could be at play.

The study was published in Human Reproduction Update.

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