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Question: From a physiological point of view, when sex is determined in a human fetus, is it equally likely to be male or female?

Studies in this area typically measure age at birth, where the data would have already been biased. There's a slight imbalance in the human sex ratio, in that there's an overall tendency towards males. However, this could be accounted for in numerous ways, such as abortion.

Fisher's principle suggests there would be an evolutionary tendency towards 50:50 chance.

{ asked by Douglas S. Stones }

ANSWER

The Fisher's principle is not applicable to the fetuses because it has been formulated for parental expenditure and basically states that the ratio of male to female parents (implying that both parties have reached the age of fertility) will tend to 1:1.

There are several mechanisms that we can use and that are mentioned in the canonical paper by James (2007):

(a) there are equal numbers of X and Y chromosomes in mammalian sperms
(b) X and Y stand equal chance of achieving conception
(c) therefore equal number of male and female zygotes are formed, and that
(d) therefore any variation of sex ratio at birth is due to sex selection between conception and birth.

James brings much evidence that none of these conventional beliefs is true. Rather, they are dependent upon many factors: exposure to stress during pregnancy, glucose level etc. He reports that there is an excess of males at birth in almost all human populations, and the natural sex ratio at birth is usually between 1.02 and 1.08. However, the ratio may deviate significantly from this range for natural reasons. (I really recommend reading the paper I linked, it is available for free).

Branum et al (2009) analyze birth statistics in the US taking into account many factors like ethnicity, gestational age and plurality and show that the ratio can increase even more with growing gestational age and has different values among different races.

So, taking everything together, I can say thay YES, the chances for human fetuses to be male are indeed higher.

{ answered by Alexander Galkin }
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