Are you destined to be a boy mom? Here's what the science says.
Studies show parents are more likely to have bigger families when kids are the same gender, creating patterns that look biological—but may be behavioral.
In theory, having a child is a coin toss: a 50 percent chance of a boy and a 50 percent chance of a girl. In reality, families stacked with only sons or only daughters appear far more often than chance would suggest.
Now, new research published in Science Advances shows these lopsided families may not be a statistical fluke. “Growing up, I noticed a pattern [of all-boy or all-girl families], and I have always wondered whether this was purely by chance, or if there was some biological underpinning,” says Siwen Wang, a researcher at the Harvard T.H. Chan School of Public Health, and one of the authors of the study.
In the study, researchers examined data from the Nurses’ Health Study II and III, which are long-running studies that have collected data from nurses over several decades, including information about the number of children they had, as well as the timing of their pregnancies. Within the study, 58,007 women gave birth to 146,064 children, resulting in an average of 2.5 births per woman.
Researchers found a higher-than-expected number of all-boy or all-girl families, which could not be attributed to chance alone. “What the data eventually returned was that it might not be as random as a coin toss,” Wang says.
Scientists are now trying to explain why. One possibility is biological—that some parents may be predisposed toward having children of one sex. The other is behavioral—that parents may keep trying until both genders are represented, skewing the numbers. Either way, the results suggest that family patterns aren’t entirely accidental—and could hold clues to the complex interplay between biology, behavior, and reproduction.
How gender preference can shape families
If biology doesn’t fully explain the trend, behavior might. In a follow-up analysis conducted by two statisticians and published as a preprint, gender preference is shown to be a major driving force behind the trends observed in the original study. “When we look in families of three or more children, then we see this over-representation,” says Judith Lok, a statistician at Boston University, and one of the authors of the preprint. “It’s not that it’s not a random draw, it’s that people keep trying until they’re all set, and have at least two genders.” These results have not yet been peer-reviewed.
(How science is helping us understand gender.)
“You can explain everything in their data just with the preferences of when parents choose to stop,” says Marcos Huerta, a data scientist and astrophysicist based in Virginia, who conducted his own separate analysis of the data. As Huerta notes, the attempts to control for gender preference, which included excluding the last child born in a family or excluding “coupon collector” families, weren’t quite enough to correct for this preference.
That preference for “one of each” isn’t new. In a 2023 study, researchers examined how gender preferences have shaped families since 1850, utilizing a combination of census data and more recent datasets.
What they found was that gender preference has always shaped family planning decisions, although it has become more pronounced in recent years. In the 1800s, families were approximately 2 percent more likely to have a third child if their first two children were the same gender. In more recent years, families are 6-7 percent more likely to have a third child if their first two children are the same gender.
“When parents start with two children of the same sex, they are more likely to have a third child,” says Todd Jones, an economics researcher at Mississippi State University, and one of the authors of the 2023 study. “This simple preference creates a surprising illusion. Even if every birth is as random as a coin toss, families with only boys or only girls are more likely to keep going, making it appear that some parents are naturally more likely to have children of one sex.”
As Jones’ research shows, this preference for having children of mixed genders goes back to the 1850s, and does not include any evidence for families preferring one gender versus another. Families with all boys are just as likely to choose to keep having children as families with all girls.
Is there a biological predisposition toward having a specific gender of child?
Although the results from this study are most likely due to parents having a gender preference, there is some evidence to suggest that parents might end up being predisposed toward having more of one gender versus another, based on biological factors. Birth ratios aren’t perfectly even: worldwide, about 105 boys are born for every 100 girls, giving boys a slight statistical edge. There is also a well-documented phenomenon, known as the “returning soldier effect,” where there is an increase in the number of boys born after major wars.
The biological reason for how these imbalances in birth ratios can arise is still unknown, although several theories exist. One theory focuses on the timing of conception, suggesting that the phase of the menstrual cycle may influence whether a boy or a girl is conceived. Some studies have found that intercourse closer to ovulation slightly increases the odds of a boy, though results have been inconsistent and remain difficult to confirm in large populations.
Another line of research looks at the pH of the uterus, which may determine which sperm cells are most likely to succeed. X-bearing sperm (which produce girls) and Y-bearing sperm (which produce boys) differ slightly in structure and resilience, and specific chemical environments could give one type an advantage over the other.
A third theory suggests that a shorter follicular stage is linked to a higher chance of conceiving boys, while a longer stage is associated with conceiving girls. The mechanism isn’t well understood, but it may relate to how hormonal changes alter the uterine environment or egg receptivity.
Other research has examined whether a mother’s physiology—such as hormone levels, glucose availability, or immune responses—could subtly influence the odds in favor of one sex. Some studies, for example, have found that higher maternal glucose levels at conception are associated with a greater likelihood of male births. In comparison, lower glucose levels have been linked to female births. A 2008 study in Proceedings of the Royal Society B reported that women with higher energy intake around the time of conception were more likely to give birth to sons. However, later studies have struggled to consistently replicate this finding.
(Pregnancy transforms the brain—and some changes last forever.)
Hormonal factors may also play a role. Elevated levels of cortisol in mothers—a marker of stress—have been tentatively associated with producing more daughters, possibly because stressful conditions make male embryos less likely to survive early development. These associations are modest, often inconsistent across studies, and difficult to confirm in large or diverse populations.
Finally, some theories shift attention to the father’s characteristics. A handful of studies suggest that men who are taller, wealthier, or exhibit higher levels of aggression may be more likely to father sons. The evidence here is especially limited and controversial, but it raises the possibility that paternal biology or lifestyle could also play a subtle role in sex ratios.
However, the evidence for why there might be a biological predisposition for giving birth to one gender versus another is still minimal, due to the complexities of conducting research on the topic.
“It is usually very hard to study human reproduction because it is not only biological, it is behavioral,” Wang says. “It is limited by how many children humans are able to produce during their lifetime, based on their economic situation, and their desire for family size.”
