Editor,—Grech et al 1 reported that sex ratio (proportion male) at birth declines highly significantly with increase in geographical latitude in Europe. I offered evidence that this may be attributable to variation of maternal hormone levels with latitude.2Grech,3 while acknowledging this possibility, proposes that the effect with latitude may be secondary to an effect with temperature. Accordingly he writes : “an interesting study would be the analysis of seasonal variations of sex ratios at birth, for individual countries. A latitude effect would thus be excluded, and any variations in the birth sex ratio would be more likely to be caused by a temperature variation effect”. If Grech were correct, conceptions in cold seasons should produce an excess of female births. I have reviewed 17 studies of seasonality of sex ratio at birth.4The results of the smaller studies …