A 1992 paper in the British Medical Journal suggested that sperm counts in human males had declined about 50 percent from the 1930s to the 1990s. Some scientists speculated that the reason might be increasing exposure to chemicals in the environment, especially so-called endocrine disrupters. However, subsequent analysis makes clear that there is no consistent evidence of a worldwide effect on human male fertility over the decades. The 1992 paper examined sperm count reports from 61 different studies published over several decades using different methods on different numbers of men and from widely different geographic locations.
Later studies find that sperm counts tend normally to vary from geographic region to region.
False trends can easily appear in data gathered under such varying circumstances.