Dr. Shanna Swan on fertility: what the evidence says · JRE #1638

What the evidence says 01 / RECORD

Swan's figures come from Levine et al. 2017 (Human Reproduction Update), a meta-regression of 185 studies covering 42,935 men, which reported sperm concentration among "Unselected" men in Western countries declining roughly 52% between 1973 and 2011 (unadjusted slope -0.70 million/mL/year), consistent with the 99-to-47 figures Swan cites. The finding remains genuinely contested rather than settled: critics note that combining studies using different counting methods and populations over five decades can introduce bias, that early studies may have overestimated baseline counts, and that publication bias could favor studies reporting declines. More recent analyses reach conflicting conclusions; a Cleveland Clinic-affiliated summary of a meta-analysis of American men found sperm concentration has been essentially stable or even rising slightly since 1970. The 2017 meta-analysis itself has not been retracted and remains widely cited by its authors, who have published a follow-up defending the methodology against these criticisms, but the underlying decline is still debated in the field. Overall, the raw numbers Swan quotes accurately reflect her own published meta-analysis, but whether they represent a genuine, uniform decline in male fertility remains an active and unresolved scientific debate.