"Sperm counts among Western men have halved in last 40 years," The Guardian reports. A major review of research carried out since 1973 found an estimated 50-60% drop in sperm count in developed nations.
Researchers looked for studies that reported measures of either total sperm count or sperm concentration in men not known to have fertility problems.
They analysed the findings of these studies and considered trends over time to see if there had been any changes in recent decades.
They concluded that total sperm count and sperm concentration had decreased over time in Western countries, but this trend was not as strong or didn't exist in other parts of the world, such as Africa, Asia and South America.
Both the researchers and the media have a number of theories as to why this might be the case, ranging from exposure to chemicals and pesticides to The Independent's suggestion that modern life was to blame.
It's not clear why. Both the researchers and the media offered a number of suggestions. But until further research is carried out, we just don't know whether these speculations have any merit.
Talk of human extinction in the media is premature. Although the study did report a dramatic-sounding decline in average sperm count from 92.8 million/ml to 66.4 million/ml, this is still well within the range needed to conceive.
Men can help protect their sperm by avoiding smoking and not drinking too much alcohol.
The study was carried out by researchers from the Hebrew University Center of Excellence in Agriculture and Environmental Health and the Ben-Gurion University of the Negev, both in Israel, as well as the Icahn School of Medicine in the US, the University of Copenhagen in Denmark, the Federal University of Parana in Brazil, and the University of Murcia School of Medicine and Biomedical Research Institute of Murcia in Spain.
The study was funded by the Environment and Health Fund, Israel, with additional support given to individual researchers from the American Healthcare Professionals and Friends for Medicine in Israel, the Israel Medical Association, the Research Fund of Rigshospitalet, the Brazilian National Council for Scientific and Technological Development, and the Mount Sinai Transdisciplinary Centre on Early Environmental Exposures.
While the press coverage did accurately report the trends, many headlines were misleading as they focused on the researchers' comments, rather than the study's findings. The actual research didn't look into the causes of any declines in sperm count.
This systematic review and meta-analysis aimed to find existing research that had looked directly at human sperm counts in different populations and explore whether any changes had occurred over time.
This study design has some benefits for exploring whether sperm count is declining, as it allowed the authors to look at findings from a far greater number of people and populations than would usually be possible in a single study.
But not all of the studies included were the same quality, and the researchers weren't able to look at data from every man involved in those studies.
The researchers searched databases of medical research in a systematic way and found 185 studies that had looked directly at human sperm count in men either confirmed to be fertile or who had unknown fertility status (unselected men).
The researchers analysed data on both sperm concentration and total sperm count collected between 1973 and 2011.
The authors also analysed data on a range of confounding factors that could have influenced sperm count, such as:
If data was missing on an important factor, the authors found ways of replacing it with an estimate.
They carried out a meta-regression analysis, where the results of the different studies were combined and the influence of other factors, such as the men's ages, was taken into account. This was an appropriate method of analysis for this type of research.
If data was missing on an important factor, researchers found ways of replacing it with an estimate.
When the researchers combined the basic results of all the studies without taking into account other influencing factors, they found that from 1973-2011 there was on average a 0.75% decrease in sperm concentration every year (95% confidence interval [CI] 0.73% to 0.77%) with an overall drop of 28.5% over the period. The average sperm count had dropped from 92.8 million/ml to 66.4 million/ml.
When they looked at total sperm count, which takes into account the volume of semen, the yearly decrease was also 0.75% (95% CI 0.72% to 0.78%) with an overall drop of 28.5%. This meant a drop from 296 million to 212 million.
When other factors were taken into account in the analysis (for example, age, region, abstinence time, sperm collection methods), the results for each group were as follows:
There were no significant changes in sperm concentration or total sperm count for unselected and fertile men from other regions.
The researchers concluded there had been a "significant overall decline" in both sperm concentration and total sperm count in Western countries over the study period, particularly among men who were unselected.
They noted there was no "levelling off" of the trend, which would suggest there may be further declines in the future.
The researchers have expressed concern at their findings, making calls for research into the causes of these trends to be prioritised.
This research presented a useful summary of existing studies in the area of human sperm count, and presented some interesting findings relating to trends over time.
But this study does have some limitations:
Although this research suggests there may be a decline in sperm count in Western countries in recent years, it doesn't offer any explanations.
It also doesn't tell us anything about the fertility of individuals, as the research was based on averages across populations.
The researchers have called for the scientific community to investigate possible reasons for the reported drop, which would seem like a good idea.