| Chapter 2: | Background on Population Sex Ratio |
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Chapter 2
Background on Population Sex Ratio
Human Secondary Sex Ratio
The primary sex ratio of sperm in mammals is 1:1, but a higher proportion of fetuses are male, possibly as a result of preferential binding of Y-chromosome sperm to the oocyte. Figure 2.1 shows estimates of the changing sex ratio over the course of gestation (Broer, Weber, & Kaiser, 1977; Kukharenko, 1973).
In mammals, the number of male live births exceeds female live births. For humans, the male proportion of births is expected to be approximately 0.513, or 103–106 male births per 100 female births (United Nations Secretariat, 1998). John Graunt was the first to compile data that showed an excess of male over female births in the 17th century (Petty, 1964, as cited in Campbell, 2001).