In 2005, the BMJ published a paper suggesting that subfertile couples were more likely than fertile couples to have boys.1 This observation was offered as support of the hypothesis that sperm bearing Y chromosomes swim faster through viscous cervical mucous. In the flurry of letters that followed, other researchers reported that their data did not support the sex ratio finding.2 Furthermore, the sperm swimming hypothesis was exposed as a persistent myth.3

In this week's BMJ, Joffe and colleagues4 pool data from several large fecundity studies, further confirming the lack of association between secondary sex ratio (boys to girls at birth) and time to pregnancy. On this point, we are confident the authors are correct. However, their hypothesis that the sex ratio could be a marker of adverse effects on the male reproductive system is less certain. They suggest that a …