Fertility rates are way down throughout the industrialized world. In Europe and Japan total fertility rates have been below replacement level (2.1 children per woman) for decades. In the United States, the rate first dipped below replacement level in the 1970s. In Denmark, the average childbearing age is actually lower today than it was in 1901.

Speculation has centered on social changes, such as women choosing to postpone birth until a later age, as responsible for the decline in fertility. While these factors may play a role, they cannot fully explain the current trends. Now researchers are pointing a finger at the harmful effects of endocrine disruptors on sperm.

Along with the decline in sperm quality, they found a huge worldwide increase in testicular cancer and a drop in the testosterone level of the average man.

“I was surprised that we found such poor semen quality among young men aged 20 to 25. The average man had up to 90% of abnormal sperm,” Niels E. Skakkebaek, lead author of a new review article on male reproductive health and worldwide fertility, said in a statement. “Normally, there would be so many sperm that a few abnormal ones would not affect fertility. However, it appears that we are at a tipping point in industrialised countries where poor semen quality is so widespread that we must suspect that it results in low pregnancy rates.”

For the review, the authors analyzed global trends in male reproductive health problems and their potential effect on declining birth rate, using information from national and international organizations such as the World Health Organization, as well as published medical research.

Along with the decline in sperm quality, they found a huge worldwide increase in testicular cancer and a drop in the testosterone level of the average man.

There is evidence that these male problems arise during embryonic development (before birth). And while both genetics and environment could play a role, the evidence tends to favor environmental exposure of the fetal testes. Moreover, the fact that these problems have increased over a relatively short time span makes it unlikely that genetics alone could explain it.

“There is no doubt that environmental factors are playing a role and that endocrine-disrupting chemicals, which have the same effect on animals, are under great suspicion. The exposure that young people are subjected to today can determine not only their own, but also their children's, ability to procreate,” explained Skakkebaek, a Senior Researcher in the University Department of Growth & Reproduction and adjunct professor in the University of Copenhagen.

The dangers of overpopulation are well known, yet too low a birth rate also poses its own risks. The current review cannot tell us how great a role male reproductive health problems play in the declining birth rate, only that they likely play a significant one. The authors urge more research to pinpoint how much of the drop in fertility is due to socioeconomic factors and how much is due to biological causes, both male and female.

The study appears in Physiological Reviews and is freely available.