Geographic variations in sperm counts: a potential cause of bias in studies of semen quality

To determine whether geographic variations in sperm counts might bias conclusions drawn from studies of semen quality. Reanalysis of published data from a meta-analysis of 61 studies from 1938 to 1990 that concluded a worldwide decline in semen quality over the last 50 years. Influence of geographic location on sperm counts. Of 61 studies in the meta-analysis, only 20 included ≥100 men. These 20 studies collectively comprised 91% of the total men studied. We focused our reanalysis on these 20 studies. Of the studies before 1970, all were from the United States and 80% were from New York. These studies represented locations with the highest sperm counts. In contrast, after 1970, 80% of the studies were from locations not represented earlier, including five studies from third world countries, where sperm counts were low. Sperm counts vary dramatically among different geographic locations. Geographic variations in sperm counts need to be considered when analyzing data from different locations.