Do Girls or Boys Really 'Run' in Families?

five boys at ball game

Everyone knows that family with five girls -- or five boys -- where the parents shrug and say, "Yup, well, girls (or boys) just run in the family!" At first glance, it sure seems like certain families are more prone to giving birth to one gender over the other. But science isn't exactly sold on the idea.


"Yes, there may be clusters of girls or boys grouped in certain families, but there is dispute among experts on the reason this may happen," explains E Scott Sills, MD, a fertility specialist at HRC Fertility.

Some scientists do say it's genetic: by combing through 973 family trees dating back to 1600, researchers at Newcastle University found that if a man produced more sons than daughters, his sons were likely to have sons as well. The researchers theorized that certain genes caused men to create more sperm with Y chromosomes (which would result in boys) while other men were genetically predisposed to make more sperm with X chromosomes (which would result in girls).

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Other researchers suggest that waves of boys or girls may boil down to the environment. In a study at Queens College examining the boy/girl ratio in China from 1929 to 1982, researchers noted an abrupt decline in boy births during the "Great Leap Forward Famine" between 1960 and 1963. Once the famine was over, boy births skyrocketed between 1963 to 1965 to compensate. The theory behind this? Boys need more resources (like food) to succeed than girls, so in situations where resources are scarce, moms are more likely to pop out girls, saving the boy births for times of plenty.

Still others claim that the mom's diet may tip the odds in the favor of one gender or the other. "There are some theories that a couple can improve the chances of having a female infant by increasing dietary intake of calcium and magnesium," says Josephine Dlugopolski-Gach, MD, an internist at Loyola University Health System. "To have a boy, they need to increase the dietary intake of sodium and potassium." 

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Yet another belief has to do with timing of intercourse: since sperm carrying the "Y" chromosome are slightly lighter than sperm with the female "X" chromosome, some argue that "Y" sperm will reach the egg faster -- that is, if the woman has ovulated recently and the egg is ready to be fertilized. But if the egg isn't ready, the slower, less fragile "X" sperm will endure longer and have a better shot at conceiving a girl.

Yet in spite of all these theories, none have overwhelmingly convinced the medical community that any one is right.

"Most studies do not support that gender runs in the family," says Dr. Dlugopolski-Gach. "You will see or hear about families with four or five kids of the same sex, but if you flipped a coin several million times in groups of five -- even though most would be a combination -- some of the tosses would come up all heads and others all tails."

In other words, families that appear to "run" in one gender are entirely within the realm of probability ... if you meet a lot of families! Unless you choose IVF and select the gender of the embryos you want implanted, your chances of having a boy or girl are still roughly what they've always been: 50/50.

Which gender seems to "run" in your family?


Image via Heather Renee/shutterstock

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