moke076/Flickr
Source: moke076/Flickr

You’re pregnant and, naturally, you’re curious if the collection of cells squatting rent-free inside your uterus will turn out to be a boy or a girl. You could always wait for the ultrasound results, but instead why not consult the long list of old wives’ tales that purport to reveal the sex of your sprog-to-be?

Rumor has is that women who prefer to sleep on their left side are more likely to have a boy. Sleeping on your right side means it’s a girl. Extreme morning sickness is also supposed to signal a girl, as are soft hands (dry hands? It’s a boy!) And my personal favorite, which manages to combine pseudoscience with sexism, is the idea that cravings for manly snacks like pretzels and chips mean you’re brewing a miniature male couch potato, and a hankering for girly treats like ice cream and chocolate signal you’ll soon pop out a little princess.

All of which is, quite obviously, garbage. Anyone with half an ounce of grey matter knows that it’s a total crapshoot. A 50/50 chance. Half the time it will be a boy, and half the time a girl.

Or will it?

A baby fit for its environment

In the 1970s, evolutionary biologists Robert Trivers and Dan Willard hatched an idea that came to be known as the Trivers-Willard hypothesis. It goes like this:

1: Females are constrained in the number of offspring they can have, because it takes a lot of time to gestate a child and then raise it to independence. Because of this, successful females don’t produce many more offspring than unsuccessful females.

2: Males, on the other hand, are unconstrained in the number of offspring they can have. If a male can find a female who will mate with him, he can have more offspring. Think Genghis Khan or George Foreman. Unsuccessful males might produce no offspring, but an uber-successful male can produce hundreds (depending, of course, on the species). Alpha-male lions or silver-back gorillas fight to control a harem of females and then monopolise mating, while their unlucky rivals are condemned to a life of solitude.

3: Therefore, it is less of a gamble to have a daughter than to have a son. Daughters will almost always be moderately successful, evolutionarily speaking. Sons may be very successful or complete failures.

4: Males in good condition are more likely to be successful, so if a mother-to-be is herself in good condition (or lives in a safe environment with a bountiful food supply) it would make more sense for her to have a son. If times are bad, a better option would be for her to hedge her bets and have a daughter.

An assumption of the Trivers-Willard hypothesis is that females can control the sex of their offspring. Bizarrely, this assumption appears to be true. Female red deer pop out more sons when they are healthy. Dominant female Barbary macaques, a type of monkey, also have more sons than daughters, presumably because sons can better reap the benefits of being born into a high-ranking family.

And humans are no exception. Parents who produce sons tend to be more physically formidable (taller, heavier, and more muscled), of higher social status, and richer than the parents of daughters.

Preferring dominant dads

All this led Jaime Palmer-Hague of Trinity Western University and Neil Watson of Simon Fraser University (both in British Colombia, Canada) to wonder if “women who are more likely to conceive sons … exhibit stronger preferences for dominance than women who are more likely to conceive daughters.”

That is, do women who think — for whatever reason — they are more likely to have a son than a daughter find themselves drawn to potential mates who are especially masculine and would therefore pass their masculine traits on to their sons?

Palmer-Hague and Watson had 66 young women predict whether they thought their first born child would be a boy or a girl. To control for the volunteers’ preferences, they were also asked what sex they would want their child to be if they were to only have one.

Palmer-Hague and Watson

A composite of men rated high for dominance (left) and low for dominance (right).

Source: Palmer-Hague and Watson

Next, the volunteers were shown pairs of male faces that differed in rated dominance. They judged whether the dominant or the submissive face was more attractive (they were not explicitly told which face was more dominant).

The results of the study showed that 58% of women predicted their first born child would be a boy, while 42% thought it would be a girl. Incidentally, this was opposite to women’s preferences: only 39% wanted a boy if it would be their only child, while 61% preferred a girl.

When it came to face ratings, women did generally prefer the more dominant male faces. Also, as predicted by the Trivers-Willard hypothesis, women’s preference for male facial dominance was related to the predicted sex of their first born child. 76% of women who predicted a boy, but only 50% of women who predicted a girl, preferred the dominant over the submissive faces.

“Taken together”, say Palmer-Hague and Watson,

these results provide support for the novel hypothesis that women choose mates based on characteristics that will be most beneficial to the reproductive success of their future offspring.

As unbelievable as it might sound, women may be unconsciously aware of the sex of child they are most likely to have, and use this information to choose a more suitable father for that child.

As the researchers acknowledge, however:

we did not seek data regarding the actual accuracy of participants’ predictions for sex of first born child, since these data would presumably take many years to compile.

So we may have to wait a while before finding out for sure if women’s predictions and preferences match up.

Reference

Palmer-Hague, J. L., & Watson, N. V. (2016). Predicted offspring sex is related to women’s preferences for dominance in men. Evolutionary Behavioral Sciences, 10(1), 10–28. Read summary

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