Was the widespread decline in sex ratios at birth caused by reproductive hazards?

There has been a decline in sex ratio at birth in recent decades in many countries. The question arises whether polluting environmental endocrine disrupters may have been responsible. It is suggested here that we are not (and will not soon become) in a position to know this because: (i) we do not know what those sex ratios would have been doing in the absence of such proposed polluters and (ii) there are plausible alternative explanations which themselves offer little prospect for testing. In short, the population sex ratio at birth seems not to be a useful monitor of reproductive hazard unless it were to change at a dramatically greater rate than has ever been reported. This is not to deny that offspring sex ratios of selected samples of workers have proved useful non-invasive indicators of reproductive hazard. However, the recent tiny recorded secular declines in population sex ratios are distracting attention from the huge and unexplained changes in other monitors of reproductive hazard, e.g. the widespread decline in dizygotic twinning rates 1960-1980 and the recent probable decline in sperm counts.