Researchers reached the strange conclusion after completing a survey of British women who had recently become mothers. They found that for every extra year a woman thought she was going to live, the odds of her firstborn being a boy increasedsignificantly.” (Guardian.UK)
It has previously been known that the male:female birth ratio increases under favorable conditions, where a population is well-nourished and without adversity, while the balance tips toward females under harsher conditions. This will be to the advantage of the survival of the population as a whole, maximizing procreative chances under stressful conditions. Having less boys than girls when one does not rate one’s chances of a long life would seem to be the psychological equivalent, or parallel, to this.
Evolutionary biologists call this the Trivers-Willard hypothesis. It suggests that when a mother feels under pressure, from anything ranging from poor health and living conditions to scarce food, it makes biological sense for her to give birth to a girl rather than a boy.
“Having females is a safe investment, in that their reproductive success is consistent,” Dr Johns said. “Boys are more risky. They are more likely to die young, and if they don’t, they have to compete for females. They could potentially provide you with lots of grandchildren, but unless you can invest in them, you run the risk of having a ‘dud’ that doesn’t produce any at all.”
But if you want to conceive a girl, should you go around meditating on a short lifespan? While there have been some studies showing that the X-chromosome-containing sperm which will give rise to a female when they fertilize an ovum are hardier than y-containing sperm, it is not as if your outlook influences which type of sperm has the swimming advantage or anything like that. It is more likely that stress levels act through producing subtle shifts in the hormonal context of the conception and gestation.
On the other hand, in the current study, it may not be that believing you will live longer produces more male offspring at all. Couldn’t it just as well be that having given birth to a boy somehow influences you in the direction of a longer estimated lifespan? In a male-centric society, moght not one unconsciously assess one’s chances of being well taken care of in old age as better if one has a son?