Why the Victorian era saw a surge in female births and war begets boys
There is empirical evidence that heavy sexual activity increases the chances that conception will occur before the most fertile time of the female cycle, as the woman may be pregnant by then. And the data also suggest that, possibly for hormonal reasons, such conceptions are slightly more likely to be boys. “It doesn’t take much imagination to suppose that the ends of wars, with servicemen home on leave or returning home, are associated with fairly intense sex – more babies were born in the UK in 1919 than any other year in history. Put all these together and you get the conclusion – frantic fornication breeds boys.”
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Which brings us back to the mysterious surge of female births in the late Victorian period. Could it be that, in the same vein in which heavy sexual activity increases the sex ratio, a trend towards sexual inactivity lowers it? “Victorian morality” distinguished itself through a set of values that espoused sexual restraint, with an increased condemnation of masturbation and sexual activity in general, repressing any form of sexuality other than penetrative intercourse. And indeed, statistics reflect a steady decline of sexual activity throughout the Victorian period, reaching its lowest point in the year 1898. But as there was less sex going on, conception tended to occur around the most fertile time of the months, bestowing a (relative) excess of baby girls on the Victorians.
{ Rolf Degen | Continue reading }
Hypothesis 1
Up to circa two decades age, it was generally supposed – but without hard supporting evidence – that pregnant women exposed to adverse environmental circumstances were at increased risk of foetal loss, and that male foetuses were at greater risk than female foetuses; and that therefore the liveborn infants produced by stressed women contained a higher proportion of daughters. That hard evidence has now been accumulated in a series of papers by Catalano and colleagues, and others.
Using time-series analysis, it has been shown that the Sex ratios at birth (SRB) briefly declined, slightly but significantly, some three to five months after many catastrophic and other adverse events e.g. the terrorist attacks on September 11th, 2001; the Troubles in Northern Ireland (1969–1998), the Breivik shooting in 2011 in Norway, the Sandy Hook shooting in 2012 in Connecticut; the assassination of President Kennedy [though the effect in this case was more marked in non-White than White births]. […]
Thus there is overwhelming evidence that sex ratios at birth are partially controlled by maternal stress-induced selective culling of frail males in utero, resulting in a conception cohort with a low sex ratio at birth. It has also been postulated that the ratio may be skewed because of fertilization of non-optimally matured oocytes under these circumstances. Moreover, it has also been hypothesized that higher coital rates will lead to ejaculation of newly formed spermatozoa cells, possibly leading to a preponderance of Y-sperm since it is also hypothesized that X-sperm age faster and are eliminated earlier.
However, it will be appreciated that selective culling of frail males during pregnancy cannot explain some of the established variations of SRB. First, it cannot explain why some reported sex ratios are higher than prevailing norms. Second, it cannot explain why these norms almost always exceed 0.5 (equal numbers of males and females).
Hypothesis 2
It has been hypothesized that human sex ratios at birth are partially controlled by the parental hormone levels of both parents around the time of conception. Ex hypothesi, high levels of testosterone (in either parent) and/or of oestrogen (in the mother), are associated with subsequent male births. And high levels of gonadotrophins (in either parent) are associated with subsequent female births. Most of this evidence is observational and correlational, and is in accordance with the hypothesis of Trivers & Willard.
{ Early Human Development | Continue reading }
