IN a recent article, Kalmus and Smith1 have attempted to explain why a sex ratio of about one half is selectively advantageous. More recently, Bodmer and Edwards2 have published a paper on the same subject. The two approaches differ in the emphasis placed on different forms of selection. Kalmus and Smith believe that there are two main selective forces : one which requires the chance of encounter of a male and a female at the reproductive age to be a maximum, and hence that the sex ratio at this age should be one half (as suggested by Crew3) ; and the other that the genetic variance of a population should be as large as possible for a given population size, which also requires a sex ratio of one half at sexual maturity. On the other hand, Bodmer and Edwards maintain that the most important selective force arises through the advantage of reproducing as efficiently as possible, that is, of making the maximum genetic contribution to future generations for a given amount of effort devoted to the bearing of young. As Fisher4 pointed out, a sex ratio of about one half is the most efficient in this sense.