Swan, SH, EP Elkin and L Fenster. 2000. The Question of Declining Sperm Density Revisited: An Analysis of 101 Studies Published 1934-1996. Environmental Health Perspectives 108:961-966.

In 1992, Carlsen et al. reported a large, global decline in sperm count for the period 1938-1990 based on 61 reports of sperm count from different countries around the world during that period. In 1997, Swan et al. reanalyzed the same data set using multivariate statistics. This reanalysis confirmed Carlsen et al., subject to constraints inherent in retrospective longitudinal studies of sperm count.

This new study increases the sample size by 2/3rds compared to the original paper, first by adding papers published subsequent to 1990, then through addition of a series of papers revealed by additional reviews of existing literature. A few of the studies used in Carlsen et al. were deleted from the analysis for reasons detailed in the current paper.

Swan et al. reason in this paper that if the finding by Carlsen et al. or by their earlier paper were a result of biases in the original sample, then adding expanding the data set by 2/3rds with new sample points should eliminate or at least reduce the statistical strength of the conclusion. In fact, the original finding is upheld, with a decine in sperm remaining highly significant statistically in the expanded data set. They conclude that the original result was not produced by a biased sample, nor dependent upon any one or a small number of data points included in the analysis.

What do they conclude?

  The current analysis suggests that the previously reported trends have continued, at least until 1996. We have also shown that the studies initially used by Carlsen et al. (1) did not represent a biased selection of the English language literature. Nevertheless, it is likely that neither this publication nor further statistical analyses of historical data will resolve the continuing debate over declining sperm counts. Critics will continue to challenge the reliability of historical data, and most will agree that residual confounding, which may be appreciable, cannot be completely eliminated.  

They go on to observe:

  The entire issue of declining sperm count has gained in importance because of the recognition of several other trends that reflect a decline in male reproductive health. Testicular cancer incidence has increased significantly for at least the past 20 years in most of the Caucasian populations that have been studied. Trends in rates of cryptorchidism are consistent with those for testicular cancer, for which cryptorchidism is a significant risk factor. These increases in rates of testicular cancer and male genital tract abnormalities, like decreasing sperm density, have primarily been seen in Western countries. Several authors have suggested that these trends, together with decreases in semen quality, may reflect a more generalized increase in testicular dysfunction. Although few of these trend studies have examined possible causes, common environmental exposures are plausible. If environmental factors have produced some, or all, of the temporal changes in sperm density, the regional differences that have been reported in semen quality, even within countries, may also reflect variation in these environmental factors.