Declining semen quality: can the past inform the present?

By using instrumentation initially designed for counting white blood cells, sperm counts have been utilized by clinicians since 1929, particularly to evaluate cases of suspected infertility. Although this basic biological parameter might be assumed to be stable over time, several studies over the past 20 years have suggested a decline in sperm count or density. The most controversial of these analyses was published in 1992. A flood of criticism followed this analysis of 61 studies that found a 50% decline in sperm density between 1938 and 1990. Critics suggested that historical methods (of counting sperm or conducting studies) were variable and unreliable, differing from modern methods both qualitatively and quantitatively. To address this issue we analyzed these studies for trends in counting methods or their variability. We found neither. Alternative analyses produced some differences in trend estimates, but statistical factors alone could not account for the total decline in sperm density. We reviewed study populations to identify trends in population characteristics, such as abstinence time, that might explain the decline. However, controlling analytically for such factors only increased the rate of decline. We conclude that historical data on sperm density, despite large random error, are surprisingly reliable. Nonetheless, understanding causes of temporal and geographic differences in sperm density must await contemporary data.