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Soc Biol. 1995 Fall-Winter;42(3-4):226-38.

Hormonal and behavioral determinants of the secondary sex ratio.

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Department of Anthropology, Arizona State University, Tempe 85287-2402, USA.


The timing of insemination relative to ovulation and the frequency of insemination appear prominently in analyses of variations in human secondary sex ratios. Explanations invoking these variables are shown to be inadequate. A new synthetic model of sex determination is proposed in which the sex of offspring is powerfully determined by the state of the cervical mucus. The cervical state is then shown to be a function of hormonal factors endogenous to the female in interaction with the effects of previous inseminations.


Increased production of estrogen estradiol tends to increase the penetrability of cervical mucus over the six or so days before ovulation. The increased production of progesterone by the corpus luteum after ovulation quickly reduces the penetrability of cervical mucus. Earlier inseminations also reduce the penetrability of cervical mucus by littering the passages through the mucus with as many as several hundred million stranded sex cells, maternally generated leukocytes, and other debris. Thus, while infrequent sexual intercourse and reduced intercourse rates in the late follicular phase increase mucus penetrability up to ovulation, frequent intercourse and an increased intercourse rate may work against any hormonally-induced improvement in the mucus. Y-bearing spermatozoa are better able to pass through the cervical os, so the sex ratio at conception is positively associated with the coital rate and does not change with time of successful insemination when coital rates are high. On the other hand, when coital rates fall or intercourse takes place once every 7-8 days (e.g., rhythm method), the sex ratio at conception does change with the time of successful insemination. The interaction of coital rate and female hormonal levels regulates the state of the cervical mucus, which, in turn, is a key proximate determinant of the sex ratio at conception. Other proximate determinants include differences in the sizes and motilities of X and Y spermatozoa. The mean proportions of Y spermatozoa in the semen are 46-47%, with much individual variation around the means. In conclusion, the state of the cervical mucus and its dependence on female hormones and coital rate explain why the sex ratios at birth vary with the time of successful insemination in cases of low coital rates. This association does not exist, however, when coital rates are high or increasing.

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