Have sperm densities declined? A reanalysis of global trend data.

In 1992 a worldwide decline in sperm density was reported; this was quickly followed by numerous critiques and editorials. Because of the public health importance of this finding, a detailed reanalysis of data from 61 studies was warranted to resolve these issues. Multiple linear regression models (controlling for abstinence time, age, percent proven fertility, specimen collection method, study goal and location) were used to examine regional differences and the interaction between region (United States, Europe, and non-Western countries) and year. Nonlinear models and residual confounding were also examined in these data. Using a linear model (adjusted R2 = 0. 80), means and slopes differed significantly across regions (p = 0. 02). Mean sperm densities were highest in Europe and lowest in non-Western countries. A decline in sperm density was seen in the United States (studies from 1938-1988; slope = -1.50; 95% confidence interval (CI), -1.90--1.10) and Europe (1971-1990; slope = -3.13; CI, -4.96- -1.30), but not in non-Western countries (1978-1989; slope = 1.56; CI, -1.00-4.12). Results from nonlinear models (quadratic and spline) were similar. Thus, further analysis of these studies supports a significant decline in sperm density in the United States and Europe. Confounding and selection bias are unlikely to account for these results. However, some intraregional differences were as large as mean decline in sperm density between 1938 and 1990, and recent reports from Europe and the United States further support large interarea differences in sperm density. Identifying the cause(s) of these regional and temporal differences, whether environmental or other, is clearly warranted.