A groundbreaking study published on Friday in the journal Science Advances suggests that a baby’s sex may not be determined by mere chance. The research indicates that the odds of having a boy or a girl resemble flipping a weighted coin, with each family possessing a unique probability. This study provides compelling evidence that an infant’s birth sex is influenced by maternal age and specific genetic factors, challenging the long-held assumption that birth sex is a random occurrence.
Historically, scientists have observed a global trend where slightly more boys are born than girls. However, this new research delves into the more intricate patterns of birth sex within individual families. The study, led by researchers at the Harvard T.H. Chan School of Public Health, analyzed data from over 146,000 pregnancies involving 58,000 nurses in the United States, spanning from 1956 to 2015. The findings reveal that some families exhibit a higher likelihood of having children of the same sex than what would be expected if each baby had an equal chance of being a boy or a girl.
According to the study, mothers with three or more children were more prone to having all boys or all girls than chance would suggest. The research proposes that sex at birth follows a weighted probability influenced by biological factors. “If you’ve had two girls or three girls and you’re trying for a boy, you should know your odds are not 50-50,” stated Jorge Chavarro, the study’s senior author. “You’re more likely than not to have another girl.”
The researchers estimated that families with three girls had a 58 percent chance of having another girl, while families with three boys had a 61 percent chance of welcoming a fourth boy. Maternal age plays a pivotal role in this phenomenon; women who began having children after the age of 28 were found to be slightly more likely to have children of the same sex. Chavarro noted that this could be due to age-related biological changes that influence the viability of the Y chromosome carried by boys, including factors like increased vaginal acidity.
The study also identified two specific genes that may be linked to the likelihood of giving birth to only boys or only girls. “While we don’t fully understand why these genes are associated with birth sex, their discovery opens up new avenues for research,” Chavarro explained. However, Iain Mathieson, a professor of genetics at the University of Pennsylvania Perelman School of Medicine, cautioned that these genetic associations should be interpreted with caution, as the genetic analysis was based on a relatively small sample size and may be influenced by other variables.
Interestingly, the researchers found that parents were more likely to have one boy and one girl than would be expected by chance, which they attribute to a common tendency to stop having children once both sexes are represented. To account for potential bias due to family planning, the researchers analyzed data after excluding each woman’s last child and also removed cases involving miscarriages or stillbirths. Even after these adjustments, the same patterns persisted, reinforcing the idea that the odds of birth sex are not random.
This study suggests that it may not be as improbable for families, such as the fictional parents in the TV sitcom Malcolm in the Middle with five sons or the Bennet family in Pride and Prejudice with five daughters, to occur. Historical instances, like King Louis VII of France, who remarried after his first two wives each bore daughters, further highlight the significance of these patterns. Chavarro emphasized that while it may take years to fully understand why some families consistently have children of one sex, this research serves as a critical starting point.
Future studies are encouraged to explore how lifestyle factors, nutrition, and exposure to environmental chemicals might influence these patterns. The researchers noted that factors such as race, natural hair color, blood type, body mass index, and height did not correlate with having children of only one sex. However, the study sample was primarily composed of nurses, predominantly 95 percent White, which may not represent the general population's health and occupational exposures.
In conclusion, this research provides evidence that the probability of a baby’s sex may vary by family, indicating that “different families are flipping different coins with different biases,” as stated by David A. Haig, an evolutionary biologist at Harvard University who was not involved in the study. This intricate relationship between genetics, maternal age, and family dynamics highlights the complexity surrounding the determination of a baby’s sex.
