Swan, Shanna H, and Eric P Elkin. 1999. Declining semen quality: can the past inform the present? BioEssays 21:614-621.

This paper analyzes several of the assumptions made in uses of existing sperm count data in studies of global trends.

"Critics [of Carlsen et al.] 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 count for the total decline in sperm density. We reviewed the 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.

What then can we conclude from these analyses? Do they suggest an average decline in sperm density between 1938-1990? The above discussion supports this conclusion. Does this imply that uniform decline in sperm density has occurred worldwide? The analyses of these studies by several authors and more recent data argue against this conclusion ....Therefore we believe that the trend summarized by Carlsen et al. reflects a significant decline in the USA (between 1938 and 1990) and northern Europe (at least post-1970), but the scarcity of data elsewhere prohibits drawing any conclusion about the rest of the world.

It must be remembered that none of these analyses have attempted to identify the causes of the trends in semen quality, although the identification of such causes is of considerable interest.

Although we cannot hope to obtain convincing evidence of exposures that might have produced the temporal trends described here, we have an indirect means of addressing this question. If indeed, environmental factors influence semen quality, these factors, unless uniformly distributed geographically, would be expected to produce not only temporal variation in sperm density, but geographic variation as well. A carefully controlled, cross-sectional study of semen quality conducted in several cities, with differing types and levels of environmental exposures, would be invaluable for assessing such geographic variation and its causes. Suppose such a study identified significant differences in sperm density between geographic areas that cannot be explained by host factors (such as age and ethnicity) and personal behaviors (such as drinking and smoking)? It might then be possible, using biomarkers of environmental exposures, for example, to point to environmental factors that have contributed to these differences. One class of chemicals that will be particularly relevant to examine are the "endocrine disrupters," those chemicals that act to alter hormone activity, particularly in offspring, when administered exposure occurs during the relevant developmental period. Several of these chemicals (methoxychlor, bisphenol A, and dioxin) are plausible candidates because very low doses prenatally has been shown to depress sperm counts in laboratory animals. A unique global study is now underway (The International Study of Semen Quality in Partners of Pregnant Women)..."