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

Objective: To determine whether geographic variations in sperm counts might bias conclusions drawn from studies of semen quality. Design: 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. Main Outcome Measures: Influence of geographic location on sperm counts. Results: 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. Conclusions: Sperm counts vary dramatically among different geographic locations. Geographic variations in sperm counts need to be considered when analyzing data from different locations.