Why siblings can be so different, according to science

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Laura Horwitz has three sons, ages 24, 21 and 17, who share the same parents and grew up in the same house attending the same K-12 schools. Still, they are “as different as can be,” said their mother, a former teacher who now owns a service that places nannies and babysitters with families in the Chicago area.

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The eldest was very studious and organized, is an out-each-night extrovert and has been pursuing a film and television career since he was a kid. Her middle child didn’t care much about academic achievement or organization, is an introvert who mostly connects with friends online, and is a bodybuilder who changed his major several times in college. The youngest enjoys cooking and performing arts, falls in the middle of the extroversion spectrum, and eschewed the team sports his older brothers participated in.

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Parents often marvel at or complain about the vast difference between their offspring, though examples of contrasting siblings, biblical, fictional and historical, abound: Cain and Abel; Snow White and Rose Red; Anne and Mary Boleyn; former president Jimmy Carter and his brother, Billy; William and Harry. When Alexandria Bishop, who proudly displayed a Go Away welcome mat, tweeted about her rainbow- and unicorn-loving sister in 2019, it became a meme.

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But when we look at the science behind siblings, perhaps we shouldn’t be so surprised how different they can be.

While it’s true that a child inherits 50 percent of their DNA from each parent, that DNA can vary wildly from child to child because it is rearranged during the reproductive process, according to Leah Burke, a pediatric geneticist and chair of the American Academy of Pediatrics’ Council of Genetics.

And although most of us don’t think about it this way, “the shuffling around is between the grandparents’ DNA as the egg and the sperm are formed,” she said. When a woman’s body creates an egg cell or a man’s body creates a sperm cell, that cell contains a mixture of the DNA they inherited from their own mother and father – and it’s a different assortment every time. (That reshuffling, or recombination, of genes is the reason for reports of siblings who get varying ancestry results from DNA test kits.)

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So siblings might inherit less-similar genetic codes than parents commonly assume. And how siblings’ genetic code manifests as traits is more complicated than previously thought.

“The majority of our traits are polygenic,” meaning they involve many genes, said John Pappas, director of clinical genetics services at Hassenfeld Children’s Hospital at NYU Langone. For example, many of us were taught in biology class that one gene affects one trait, but eye color, for example, involves at least 16 genes and many different regulatory genomic regions.

And the possibilities for variance between siblings’ DNA don’t stop there, Pappas said. Something called epigenetics is at work. It involves factors such as diet, environmental pollutants and stress that can change the ways genes are expressed. Because of epigenetics, even genes that are shared between siblings might generate different results.

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Circumstances such as prenatal exposure to alcohol or being born prematurely can “temper the genetics by quite a bit,” Burke said. While she believes that further research will prove that many more traits are more heritable than they are now considered, “What you do with your genes is very different from one person to another, and that’s largely environment.”

The longtime debate over which matters more in raising children – nature or nurture – is being altered by the realization that nature and nurture work together to influence how our children develop.

One of the ways we know that is through studies of identical twins. Research such as the landmark 1990 “Minnesota Study of Twins Reared Apart” have found similarities in intelligence, religiosity, interests and behavior in identical twins that suggest a strong genetic component. But by looking at identical twins raised together, researchers have also found differences they attribute to environmental experiences.

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For example, a 2004 study led by Avshalom Caspi, now a professor of psychology and neuroscience at Duke University, found that differences in maternal emotions toward 5-year-old identical twins affected their behavior. In a recording from that study, which involved twins born in England and Wales, a mother can be heard describing one identical twin daughter as “one little cow . . . she’s just boisterous, naughty, yeah, just a complete and utter pain in the backside . . . that’s all I can say about her really.” She describes the other child as more obedient, placid, loving and says she misses her more than her sister when she is gone. She “wouldn’t change anything” about that child.

In the families, “the child who experienced more warmth, sensitive parenting from their mother displayed fewer behavioral problems and also had fewer symptoms of depression and anxiety,” said Jasmin Wertz, a psychologist at the University of Edinburgh who is continuing to follow these twins. “And because these children are genetically identical, there does seem to be an effect of the environment, specifically of the differences in parenting that these children experience.”

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The study found a number of possible reasons that mothers responded differently to their identical twins, including the fact that one child had more health problems than the other, one remained in the NICU longer or one reminded them more of themselves or of an ex-partner.

Other areas of parenting that seem to lead to dissimilarities in identical twins are cognitive stimulation (children who receive more of it do better educationally) and discipline (children who are disciplined more harshly are more likely to develop behavior problems), Wertz said.

If this is the case with identical twins, you can see how the interests, achievements and personalities of siblings who are genetically different could be reinforced by their parents and shaped by their environment until they hardly resemble one another. As physician and author Gabor Mate put it in a viral TikTok excerpt interview, “No two children have the same parents” because the parents are at different stages in their lives when their children are born (except twins) and have different responses to each child.

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Meanwhile, differences between siblings might not just be something parents should wonder about, but something they should encourage. While you want to parent as equally as possible in terms of providing warmth, discipline and varied opportunities for cognitive stimulation, you also want to leave space for them to express their individuality, said Wertz, who has two young daughters.

“I think it is a big task that you have as a parent, to get to know your child,” she said. In addition to accepting and learning how they are different from their siblings – and that the parenting strategy you developed for one might not work at all with another – you also have to accept how they are different from you. “Sometimes we like the things that are similar in our children to us,” Wertz said. “We can identify with them more. It’s much easier to parent them when they’re doing things that we also remember ourselves doing as children or what we do as adults.”

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Horwitz, the mother of three very different sons, said the fact that they were also different from her and her husband was another surprise. “I did think that more of my husband and I would rub off on them and kind of take our lead in things. And they really each very independently came up with their own likes and dislikes.”

Wertz said she tries to approach her two young girls judgment-free to respond to their individual personalities and interests. She looks for activities that she thinks they might enjoy based on what she knows about them – and finds both exciting and gratifying when it works out.

Horwitz learned this the hard way. “It took me a long time to admit that my middle guy was much more of an introvert,” she said. “I kept pushing, ‘Don’t you want to have friends over,’ until I finally came to realization that he was just fine interacting with them online, and wasn’t necessarily as interested in being as social as myself or my oldest.” When she stopped pushing, her relationship with her son improved. And she was much quicker to get onboard when she realized her youngest son wasn’t into team sports. Together they looked for another kind of physical activity, and he has gotten a second-degree black belt in karate.

While figuring out how to deal with her sons’ individual personalities and interests was sometimes challenging when they were younger, she said, now “I can celebrate who they are.”

The bottom line is that “everyone’s life experience is different – and that is from the get-go,” Burke said. “Giving your children space to be different is just extremely important.”