Future inseminations may look more like this than the past reality where there were a lot more sperm approaching the egg. Christoph Burgstedt/Shutterstock

By collecting every credible study they could find, a team of international researchers have confirmed that men in Europe, North America, Australia, and New Zealand are producing less than half the sperm that the previous generation made at the same age. Despite some scaremongering headlines, this won't send the human race extinct, but there are some disturbing possible causes.

The idea that male fertility is declining hit the headlines in 1992 as a result of a Danish study reporting that semen quality had fallen dramatically in recent decades. However, in the years that followed, many studies were published with wildly contradictory results. The problem was that while people could take accurate measurements now the need was clear, earlier data was patchy and unreliable, making it difficult to confirm a change.

In the journal Human Reproduction Update, an international team have drawn together 185 studies, featuring 42,935 men, to give a sample so comprehensive they consider it conclusive, at least for the developed nations.

During the period 1973-2011, sperm count declined by 59 percent and sperm concentration by 52 percent in the nations the authors categorized as “Western”, after controlling for factors such as age and collection method. Unsurprisingly, many more men are producing sperm quantities so low they meet the threshold for infertility. Moreover, this was not a sudden drop, but a steady decline of 1.6 percent a year for total sperm count, which has not slackened since 1993. Presumably the count has fallen further since 2011, although the researchers didn't look at more recent measurements.

There were too few studies from Africa, Asia, and South America, particularly towards the start of the period, for the authors to make confident pronouncements, although it looks like a slower decline is occurring there.

The availability of in vitro fertilization (IVF) and other fertility treatments mean reduced male fertility is unlikely to have a major impact on overall birth rates, which have declined primarily through choice. It’s possible, however, that lower sperm counts are contributing to the recent reduction in unplanned pregnancies in the United States.

Much more worrying, however, are the possible reasons. First author Dr Hagai Levine of the Hebrew University of Jerusalem said in a video: “This study is an urgent wake-up call to investigate the causes of the sharp ongoing drop in sperm count, with the goal of prevention.”

Meanwhile, many theories have been put forward, including obesity, hormone disrupting chemicals in the environment, parental smoking during pregnancy and childhood, and intergenerational exposure to pollution. Some of these are also likely to be affecting animals, who don’t have the same access as humans to assisted reproduction. Moreover, since sperm count is inversely related to mortality, the trend might point to wider problems.