UI 1, XY 0

Unemployment favors girl babies over boys

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“Boys will be boys”. A timeworn idiom accounting for the irritating mischief perpetrated by those of us sporting testicles.  And who’s to deny it – what else would boys be after all? Well, it seems that if the economy tanks and mass layoffs ensue, boys may turn out to be girls! Ok, perhaps not literally. What we’re talking about is the greater likelihood of a baby being a girl as an effect of a rapid increase in unemployment insurance (UI) claims. Seriously.

Ralph Catalano, a professor of Public Health at the University of California (Berkeley), and his colleagues have recently suggested that economic contraction reduces the number of boys at birth, changing what is known as the secondary sex ratio. This ratio is calculated as the number of boys born divided by the number of girls: if greater than 1.0, then boy babies outnumber girls, and if less than one, then girl babies rule! For the majority of human societies, this ratio slightly favors boys, hovering around 105 boys for every 100 girls. [In case you were wondering, there are other sex ratios calculated by demographers (people who study population dynamics): primary refers to the relative proportion of the genetic combinations of XX (♀) versus XY (♂) at conception, tertiary denotes the number of post-pubescent males and females, while quaternary sex ratio is the number of men and women aged 65 and over – as you might guess, this last one really favors longer-lived women.]

Knowing that stress can affect a number of aspects of human physiology, Catalano’s group looked at the pattern of births in boys and girls in California with regard to the timing of UI applications following notification of impending mass layoffs – unquestionably a stressful situation. (Mass layoffs are defined by the US government as 50 or more claims originating from one employer over a five-week period.)  They found that, with each peak in the number of UI claims, the sex ratio declined in the following two months – more girls were being born than expected. In fact, over a five-year period, mass layoffs leading to higher than anticipated unemployment given normal economic cycles resulted in the ‘loss’ of over 3000 boy babies from the California population.

So how do we account for the apparent relationship of economic contraction predicting a female-biased secondary sex ratio? Catalano and coworkers suggested two possible routes to this outcome. Both ideas relate to the level of stress in one’s social or physical environment (such as loss of household income), neither is very well understood. One possibility is that stress affects the primary sex ratio; in other words, conception favours girls (XX). This could occur if stress reduced the number or quality of a man’s Y-chromosome carrying sperm or reduced the frequency of intercourse. (Statistically speaking and for reasons not yet appreciated, having sex less often tends to result in more girls being born.) And if you think about it, stressful situations such as wars, famine, earthquakes and the like are not great libido boosters!

The second path, favored by Catalano, relies on a mechanism known as ‘prenatal selection’ by which a woman can terminate a fetus in utero. This would happen in cases where her lot in life is improved only when all her offspring are likely to survive birth, live to maturity, and produce grandchildren for her. In evolutionary terms, thinking back to when our distant ancestors roamed a harsh landscape eking out a living hunting this and collecting that, such a system would favor birth of robust males capable of reaching adulthood. That is, real men able to ‘bring home the mammoth’. Giving birth to weaker, frailer males would not be a benefit to our ancestors living in small hunting and gathering social groups, unless conditions were particularly kind and life’s stressors considerably reduced. Likely not often!

So maybe we should not be surprised that the frail males would be culled by mom while still in her womb? Other research has shown that men born when the sex ratio favors women tend to live longer, healthier lives, suggesting that they were in fact more robust as babies. It would seem then, that a reduction in sex ratio to favor females in times of economic contraction and job loss may reflect a hold-over from a time in our evolutionary past when a not particularly robust male baby, whose birth would pose a liability to its small band of hunters and gatherers, was effectively given the kibosh by mom.

Where to find the science:

Catalano R. et al. (2010) Selection in utero: A biological response to mass layoffs. American Journal of Human Biology 22:396–400. https://DOI10.1002/ajhb.21011 

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