The authors offer some propositions which I take to be logically flawed. These may be summarized as follows:
(A) the authors write in their abstract `(1) sex determination is random, or (2) sex determination is facultative or biased', (B) the authors claim (1) there is evidence against Weinberg's Rule--that among human dizygotic (DZ) twin pairs, there are almost exactly equal numbers of same-sexed (SS) and opposite-sexed (OS) pairs--and that (2) this evidence constitutes grounds that support `the biased or facultative position' (their p. 169).
I shall deal with these points in order.
In this context the word `facultative' may be taken to mean "tending to confer reproductive advantage as e.g. in producing more grandchildren." There are many examples of offspring sex ratios of subpopulations which differ significantly from those of their parent populations, such as the offspring sex ratios of men exposed to a number of deleterious chemicals (e.g., dioxin: Mocarelli et al., 1996; and boron: James, 1999); or men destined to suffer from various diseases such as prostatic cancer (James, 1990) or testicular cancer (Moller, 1998). However there are no grounds for supposing that these sex ratio biases are facultative: indeed it is invalid to infer (without further argument) a facultative function for an established sex ratio bias. The point has been satirized by Gould & Lewontin (1979). It is only from documented reproductive advantage that one may infer facultative sex ratios.
Most of the data in the authors' Table 1 was reviewed in the most recent study cited in that table (James, 1992). It is worth explaining why the evidence in that table (which at first sight looks overwhelming) remains indecisive.
In estimating the proportion of DZ twins which are SS and OS, it is usual to observe the following rules in regard to a randomly ascertained sample of twin pairs.
(1) All opposite-sexed twins pairs are DZ.
(2) For practical purposes, all monochorionic twin pairs are MZ.
When these two categories of twins are set aside, one is left with the subsample of same-sexed dichorionic pairs. These may be either MZ or DZ. These twins of unknown zygosity are then subjected to a series of tests by genetic markers (e.g., of blood groups). Each pair which is discordant on any marker is diagnosed DZ. However, after the testing, the remaining twin pairs (which are concordant on all markers) contain all the MZ pairs plus some undiagnosed SS DZ pairs which (by chance) happen to be concordant on all the markers so far employed. It is, in principal, possible to estimate this number of concordant SS DZ pairs by applying the same set of markers to the opposite-sexed pairs (which are known to be DZ). However, though the OS twins in many of the cited studies were tested on such markers, it is clear that this testing was, in general, not so rigorous (viz on not as many markers) as that on the SS pairs. This is for a good reason. The main purpose of the markers is to establish whether a pair of twins is MZ or DZ. But that is already known in the case of OS pairs! Hence the number of SS DZ pairs concordant for all markers tend to be over-estimated.
So, pace Coney and Mackey, no firm conclusion may be reached from the data in their table. With modern genetic techniques, the relative numbers of SS and OS DZ twin pairs will be established--but, as far as I know, accurate evaluation of the ratio has not yet been published.
In short, there is, as yet, no compelling evidence against Weinberg's Rule. And even if there were, that would not affect the status of the proposition that (some) sex ratio variation is facultative. The two propositions are logically independent as I noted (James, 1997a).
I should like to make some observations on whether (some) human sex ratio variation nevertheless is facultarive and, if so, whether it is controlled by the "woman as final arbiter."
Coney and Mackey (1998) cite a number of papers suggesting facultative variation of sex ratio in a number of nonhuman species, and it would seem likely that there is some such variation in human beings too. However, I would suggest strongly that some human sex ratio variation is not facultative but is incidental to other processes. There is now very substantial evidence that mammalian (and among them, human) sex ratios at birth are partially controlled by the hormone levels of both parents around the time of conception (James, 1996). The relevant hormones include testosterone, estrogen, and the gonadotropins--which themselves are markers of health. For that reason there is some initial suspicion that in general, human sex ratio variation would reflect the health of parents and thus have facultative consequences. However the profiles of these hormones differ with different disease processes. Consider the T/G ratio where T and G are a man's standardized levels of testosterone and gonadotropin. This ratio is high in men destined to suffer prostatic cancer (Bosland, 1988) and low in men destined to suffer testicular cancer (Petersen et al., 1999). In conformity with my hypothesis, these men reportedly have significant excesses respectively of sons (James, 1989) and daughters (Moller, 1998). Yet in the absence of direct testing, it would seem unlikely (if only because of their opposite skews) that these sex ratios are facultative. Rather they would seem incidental to a general process which has overall facultative functions.
As to whether the woman is the "final arbiter," the examples cited above (of sex ratio variation with paternal diseases and chemical exposures) suggest that the father makes a direct contribution to the "decision" about the sex of offspring. Accordingly, I have suggested a mechanism for sex selection in which both parents have a role (James, 1997b).
William H. James
Bosland, M. C. (1988). The etiopathogenesis of prostatic cancer with special reference to environmental factors. Advances in Cancer Research, 51, 1-106.
Coney, N. S., & Mackey, W. C. (1998). The woman as final arbiter: A case for the facultative character of the human sex ratio. The Journal of Sex Research, 35, 169-175.
Gould, S. J., & Lewontin, R. C. (1979). The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proceedings of the Royal Society of London `B', 205, 581-598.
James, W. H. (1989). Parental hormone levels and mammalian sex ratios at birth. Journal of Theoretical Biology, 139, 59-67.
James. W. H. (1990). The hypothesized hormonal control of the human sex ratio at birth -- an update. Journal of Theoretical Biology, 143, 555-564.
James. W. H. (1992). The current status of Weinberg's differential rule. Acta Geneticae Medicae et Gemellologiae, 41, 33-42.
James, W. H. (1996). Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels at the time of conception. Journal of Theoretical Biology, 180, 271-286.
James, W. H. (1997a). Weinberg's rule and facultative sex ratios. Mankind Quarterly, 37, 437-441.
James, W. H. (1997b). A potential mechanism for sex ratio variation in mammals. Journal of Theoretical Biology, 189, 253-255.
James, W. H. (1999). The sex ratio of offspring of people exposed to boron. Reproductive Toxicology, 13, 235.
Mocarelli, P., Brambilla, P., Gerthoux, P. M., Patterson, D. C., & Needham L. L. (1996). Change in sex ratio with exposure to dioxin. Lancet, 348, 409.
Moller, H. (1998). Trends in sex ratio, testicular cancer and male reproductive hazards: Are they connected? A.P.M.I.S. (Acta Pathologicae, Microbiologicae et Immunologicae Scandinavica, 106, 232-239.
Petersen, P. M., Skakkebaek, N. E., Vistisen, K., Rorth, M., & Giwercman, A. (1999). Semen quality and reproductive hormones in men with testicular cancer. Journal of Clinical Oncology, 17, 941-947.…