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

Last year Swan et al. (1) published a reanalysis of data from 61 studies originally compiled and analyzed by Carlsen et al. (2). Just prior to the appearance of the Swan et al. artide, we published a reanalysis in another journal (3). Regional differences were considered in both reanalyses, but we examined only the effect ofyear in the final models (fertility status was also considered in the initial model), whereas they included several additional indicators culled from each study. However, while the results in the two papers for the U.S. studies were very similar (coefficients for the effect ofyear of-1.3 and-1.5 in their paper and ours, respectively), Swan et al. (1) reported a significant decline in sperm counts over time for Europe, whereas we found a nonsignificant decline. We doubted that this difference was due to the confounding with the additional covariates that they induded, so we decided to explore. We found the reason for the difference to be that Swan et al. only did a reanalysis of a subset of studies from the Carlsen et al. compilation (2). While dropping "two studies that included men who conceived only after an infertility workup" (1) seems justified on scientific grounds, dropping three non-English language studies was arbitrary, inappropriate , and led to the different results. Two of the three non-English papers were from Europe and were written in Danish and German in decades before English dominated the scientific literature as it does today. These two studies, contrary to an assertion of Swan et al. (1) in their discussion , have sperm count values that are low relative to later studies done in Europe, so the slope is nonsignificant when they are