The march to mannishness begins at conception with the pairing of the X and Y chromosomes.
That sex is determined at conception is a relatively new idea. Chromosomal determination of sex was first co-discovered by Nettie Maria Stevens and Edmund Beecher Wilson in 1905. Prior to Stevens and Wilson, it had been assumed across the millennium that sex was a function of environmental factors and that woman was the lesser of man.
Typical for the time, and in their book The Evolution of Sex, Geddes and Thomson (1889) argued that we start sexually undifferentiated and across time develop into either male or female. This development can occur early or late in pregnancy and is a result of specific factors, such as nutrition, seasonal temperature, and age of the parents who are attempting to conceive. It was believed that, generally, harsh environments lead to more males, whereas softer and more forgiving environments result in a greater number of females.
There is no doubt that the environment, be it physical or cultural, directs the development of children and how those children eventually define what it is to be boy or girl, woman or man. It, therefore, made sense to assume that the social environment could also extend backwards into gestation and influence the sex of the unborn.
However, as argued by Stevens and Wilson, no matter how potent the immediate environment may be, it has no influence on whether you were genetically born male or female. That, my friends, is a cosmic crap-shoot.
- Geddes, P., & Thomson, J. A. (1889). The evolution of sex. London: Walter Scott.
- Wessel, G. M. (2011). Y does it work this way? Nettie Maria Stevens (July 7, 1861–May 4, 1912). Molecular Reproduction and Development, 78(9), Fm i.