SPENCER WILSON
The year was 1974. As newspaper columnists were busy wringing their hands over the development of in-vitro fertilisation and the prospect of “test-tube babies”, an alarming discovery about male fertility completely escaped their notice.
Comparing contemporary semen samples with historical data, two doctors in Iowa named C. M. Kinloch Nelson and Raymond Bunge showed that there had been a drastic shift in men’s sperm counts over the previous two decades. In 1951, each millilitre of semen contained 107 million sperm; by the 1970s, that figure had dropped by more than 50 per cent – to just 48 million. The average volume of semen from a single ejaculation had fallen too.
By the 1990s, the issue started catching considerably more scientific attention, although some researchers were still sceptical. They blamed differences in techniques, or the fact that studies were mostly on men already having treatment for infertility. Such doubts are now shrinking. “There is a huge body of scientific evidence showing this decline,” says Albert Salas-Huetos at the University of Rovira i Virgili in Spain.
For researchers like Salas-Huetos, the big question is no longer whether this “spermageddon” is really happening, but why and what to do about it. Studies are beginning to shed light on environmental toxins that may be to blame, as well as other lifestyle factors contributing to the problem. With a better idea of the prime suspects, we may finally be able to put the brakes on this trend, or even reverse it.
Globally, around 1 in 6 people have trouble conceiving, according to a recent report by the World Health Organization. There are many potential causes, but between 30 and 50 per cent of cases are linked to problems with the quantity and quality of semen. It may be that the total number of sperm is simply too low, or that the tadpole-like cells struggle to swim – a problem called poor motility – which vastly reduces the chance that sperm can reach the ovum, or egg cell. Some may have genetic defects within the chromosomes they are carrying, known as DNA fragmentation.
Shanna Swan, a reproductive epidemiologist at the Icahn School of Medicine at Mount Sinai, New York, has led many of the most eye‑catching studies. Her interest began in the 1990s, when she was asked by the US National Academy of Sciences to independently review a study from Denmark reporting rapid sperm decline. Swan was initially sceptical: she suspected that the researchers might have missed some confounding factor in their analysis. When she crunched the data, however, she kept finding the same rate of decline predicted by the Danish team. “We didn’t change the slope at all – not down to the second decimal place,” says Swan.
Her conviction has only increased over the subsequent decades. In 2017, she and her colleagues published a meta-analysis that considered data from 185 studies of more than 42,000 men between 1973 and 2011, making it the largest of its kind. Swan’s team examined two different measures: the concentration of sperm in a millilitre of semen and the total number of sperm in the sample. In North America, Europe, Australia and New Zealand, both figures seemed to be falling at a rate of around 1.5 per cent per year on average, resulting in a 50 to 60 per cent drop over the whole period.
If the decline continued at this rate, the median sperm count would reach zero by the mid-2040s. Within a generation, “we may lose the ability to reproduce entirely”, the magazine GQ declared in 2018.
At the time of this analysis, Swan and her team didn’t have enough data to draw strong conclusions about sperm counts in the rest of the world. They have now filled this gap in knowledge with additional data from South America, Asia and Africa. The ensuing paper, published in 2023, reported a decline on every continent studied.
Such studies do have some limitations. Meta‑analyses can be skewed by differences between datasets. Counting sperm is a fiddly job and the technology used to do it has changed over the years, which may bias the reported numbers. Nevertheless, the latest studies control for this potential bias and the pattern remains, says Richard Lea at the University of Nottingham, UK.