Response To James - Brief Article

Journal of Sex Research, Nov, 2000 by Wade C. Mackey, Nancy S. Coney

The title of our article is: "The woman as final arbiter: A case for the facultative character of the human sex ratio."

The (primary) human sex ratio can be considered either to be a function of purely random chance, such as a 50/50 split, or to be a function of systematic responses to environmental events, such as the odds of conceiving a son (XY) versus daughter (XX) are biased. Weinberg's Rule argues for independence (of sex) for each conception. Facultative means adaptive responses to varying environments. Accordingly, even though primary sex ratios are difficult to measure, if (secondary) sex ratios are found which differ from a 50/50 split, then Weinberg's Rule becomes suspect. If a theory can be aligned with the biased sex ratios, then the "independence" upon which Weinberg's Rule is founded also becomes suspect. We presented five separate data bases which reflect a skewed (secondary) human sex ratio. We also present theories by Fisher (1930) and by Trivers & Willard (1973) which would successfully predict the direction of the bias.

Their theories suggest that the mother-to-be "reads" her environment, and the "reading" would tend to bias a conception/birth more toward a daughter or more toward a son. Given the mammoth differentials in investments between the father and the mother in the gestation of the child and in the nursing of the child and in the (primary) caretaking of the child, the physiology of the woman would be the better candidate to execute the bias rather than that of the man. It is difficult to make the reverse case.

James writes "In this context the word `facultative' may be taken to mean `tending to confer reproductive advantage as e.g. in producing more grandchildren'." Our use of the word "facultative" is its definition, not any implication: to wit--"adaptive responses to varying environments." James then proceeds to give evidence for a facultative human sex ratio. He cites chemical agents aligned with biased sex ratios. He cites diseases aligned with biased sex ratios. He cites hormone differentials which are aligned with biased sex ratios.

James argues that our survey of studies which illustrate the over-representation of like-sex dizygotic twins (DZ) versus unlike-sex dizygotic twins (DZ) could be explained by systematic false positives (i.e. like-sex DZ are overestimated). Such might be the case for each of the 19 of 23 studies which did show a ratio of like-sex DZ to unlike-sex DZ to be greater than 1.00. The mean ratio of the sample of 23 studies was 1.24 (p [is less than] .01; 2-tailed). That is a lot of false positives.

If the twin study is waived aside, that still leaves the world wide (secondary) sex ratio of approximately 105 males born per 100 females (the primary sex ratio is invariably viewed as higher than 105; how much higher depends upon which author is cited), the sample from the Who's Who survey, the sample of U.S. President's children, and the sample of polygynous Mormons of-the 19th century. These sex ratios are biased. The bias occurs in the direction that the Trivers and Willard and the Fisher theories would predict.

James suggests that both parents have a role in the mechanisms for sex selection. We agree. But we suggest that the father's influence is indirect in that he influences the mother. It is difficult to conceptualize the man's input at or beyond the fallopian tube. He is part of her total environment which she "reads." The mother is more direct and has last "tags." She is the final arbiter.

Wade C. Mackey
Nancy S. Coney

REFERENCES

Fisher, R. A. (1930). The genetical theory of natural selection. Oxford, England: Oxford University Press.

Trivets, R. L., & Willard, D. E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science, 179, 90-92.