Editor,—Grech et al 1 report that sex ratio (proportion male) at birth declines highly significantly with increase in geographical latitude in Europe. They wonder if this is an effect of temperature variation. This may be so, but I suggest that variations in maternal hormone levels are a proximate cause. The reason for suggesting this is as follows.

I have reported highly significant correlations between latitude and (a) birth weight and (b) maternal age standardised dizygotic (DZ) twinning rates across the countries of Europe and the states of the United States. Birth weight and DZ twinning rates are both higher at more extreme latitudes.2

There is direct3 and indirect4 evidence that maternal oestrogen levels correlate with the birth weights of their infants. Moreover maternal oestrogen levels reportedly correlate with the probability of bearing a pair of DZ twins.5Lastly, there is good evidence that maternal hormone (including oestrogen) levels at the time of conception partially control the sexes of offspring.6 Bearing in mind Occam's Razor, it is tempting to propose that one cause (variation in maternal hormone levels) is at least partially responsible for all three effects—including the variation of sex ratio with latitude.