The results of a landmark study on male fertility published in the British Medical Journal in 1992 generated worldwide concern by suggesting that male sperm counts had dropped 50% over the last 50 years.
However, the report reviewed 61 studies of semen quality carried out between 1938 and 1990, which were not performed identically and may have lacked important controls.
Unfortunately, the publication became so popular upon its release, that its methods may not have received a thorough analysis by the mass media.
Here are a few of the methodological issues with the report:
Volunteers may have been primarily infertile, as these men would be more likely to involve themselves in a study on fertility
The length of time between testing the sample and the previous ejaculation was not accounted for;
Methods of sperm analysis has changed over time, with some techniques relying on individual judgement;
Some of the original studies were not comprehensive enough to be representative of a population;
Regional and geographic differences were not accounted for.
Thought the 1992 study reported on a number of studies on male fertility, a new large, strongly controlled study would provide more accurate information on worldwide male fertility levels.