Trends in global semen parameter values.

llegations for a worldwide decline in semen parameter values have not withstood scientific scrutiny. Methodological flaws in an influential 1992 paper are summarized here, and studies that have been published since 1992 are reviewed. Of the 35 major studies of time trends in semen quality reviewed here, eight (a total of 18 109 men) suggest a decline in semen quality; 21 (112 386 men) show either no change or an increase in semen quality; and six (26 007 men) show ambiguous or conflicting results. The cause (or causes) of the geographical and temporal variations in semen parameter values reported by these diverse studies deserve further investigation. This paper critically examines two related scientific assertions: that there have been worldwide declines in semen quality in recent decades and that these declines are related to environmental exposure to minute levels of ‘endocrine disruptors’ (chemicals that exhibit some degree of estrogen-like activity in the body). The data supporting these assertions will be demonstrated to be weak, at best. Reported declines in semen parameter values are likely to be either highly local phenomena with an unknown etiology or the result of methodological errors arising from attempts to observe highly variable physical attributes (semen characteristics) with relatively lowresolution tools (retrospective analysis of nonrandomized study populations).