A British study indicates that women who believe they will live long lives and possess optimistic personalities are highly likely to give birth to boys.
Researchers found that the more women felt they would live long lives, the greater the probability of their first child being male.
The findings suggest that, at least in some regions, people's outlook on the future is influencing the sex ratio of the local population.
Previously, researchers observed that women who were physically healthy, lived comfortably, and consumed ample food typically gave birth to sons, while impoverished women often had daughters.
Published in Biology Letters, this latest study surveyed 609 mothers in Gloucestershire. One question asked, "How long do you expect to live?"
Women from lower-middle-class backgrounds and working-class families estimated their lifespans at 40 years, while others believed they could live to 130.
Dr. Johns suggests the link between projected longevity and the sex of a woman's first child can be attributed to a "selfish" behavior within human genes.
Evolutionary biologists call this the Trivers-Willard hypothesis. It suggests that when a mother faces stressors—such as poor health, harsh living conditions, or food scarcity—biological cues may lead her to conceive a girl rather than a boy.
However, this recent finding does not imply women can alter their first child's gender by changing their outlook on the future.
Dr. Johns stated that the findings do indicate a link between infant sex and maternal personality. "You can observe this in modern populations, which is quite astonishing."
Evolutionary psychologist Robin from the University of Liverpool noted that while the research is intriguing, biologists have yet to fully understand why this phenomenon occurs.