CBS Local – A new study shows that the sperm count in men has been cut in half over the last 40 years in western countries. More alarming than the steep drop, the scientists who published their findings don’t know the reason for the decline.

The study followed 43,000 men from 1973 through 2011 from western societies in North America, Europe, and Australia. Researchers found that men’s sperm count fell by an average of 1.4 percent each year; a stunning decrease of 52 percent overall.

“The results are quite shocking,” said Hagai Levine. Levine is the lead author of the study and an epidemiologist at the Hebrew University of Jerusalem. “This is a classic under the radar huge public health problem that is really neglected,” he added.

The international study, published in the journal Human Reproduction Update, suggested that the drop could be a result of poorer health among men. Researchers pointed to body weight, lack of physical activity, and smoking as possible links to the decline.

However, no positive proof was found.

Critics of the study and previous sperm count findings say that the tests don’t accurately account for men suffering from infertility problems. Studies done in other countries and in different age groups also differed with Levine’s group, however there were far fewer tests conducted with those groups in recent years.

“We have got very little epidemiological evidence to say what might be causing it, so we are still scratching our heads a little bit in my view,” said Allan Pacey, professor of andrology at the University of Sheffield.