Elizabeth Chang
THE WASHINGTON POST – 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.
The oldest child was extremely organized and disciplined. He has been an avid follower of a television and film career since he was a child. Her middle child didn’t care much about academic achievement or organisation, is an introvert who mostly connects with friends online, and is a bodybuilder who changed his major several times in college. The youngest is a performer and cook, and falls somewhere in the middle of the extrovert spectrum. He also shuns the team sports his older brothers took part in.
Parents are often amazed or frustrated by the differences between their children. But there are many examples of contrasting siblings. These include Cain and Abel, Snow White and Rose Red, Anne and Mary Boleyn, former president Jimmy Carter and his brother Billy, and William and Harry. Alexandria Bishop, a proudly displayed Go Away welcome rug, posted about her rainbow-loving sister in 2019. It quickly became a popular meme.
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 per cent of their DNA from each parent, that DNA can vary wildly from child to child because it is re-arranged during the reproductive process, according to pediatric geneticist and chair of the American Academy of Pediatrics’ Council of Genetics Leah Burke.
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. This is why siblings may have different DNA test results.
Siblings might inherit genetic codes that are less similar to those of their parents. 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 director of clinical genetics services at Hassenfeld Children’s Hospital at NYU Langone John Pappas. Many of us learned in biology class that one gene can affect one trait. However, eye colour is a result of at least 16 genes and many regulatory genomic regions.
And the possibilities for variance between siblings’ DNA don’t stop there, Pappas said.
Epigenetics is at play. Epigenetics is a process that alters the way genes are expressed. It includes factors like diet, stress, and environmental pollutants. Epigenetics can make it possible for genes that are related to siblings to produce different results.
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 realisation that nature and nurture work together to influence how our children develop.
Study of identical twins is one of the best ways to prove this. Research such the 1990 landmark Minnesota Study of Twins Reported Apart Similarities in intelligence, religion, interests, and behavior have been found in identical twins, which suggests a strong genetic component.
However, identical twins have been compared to find differences that researchers attribute to their environmental experiences.
A study conducted by Avshalom Cassi, now a Duke University psychologist and neuroscience professor, in 2004 found that mothers’ emotions towards identical twins five years old 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 her other child as being more patient, loving, and obedient than her sister. She “wouldn’t change anything” about that child.
In the families, “the child who experienced more warmth, sensitive parenting from their mother displayed fewer behavioural problems and also had fewer symptoms of depression and anxiety,” said a psychologist at the University of Edinburgh Jasmin Wertz, 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.”
There were a variety of reasons why mothers responded differently to identical twins. One reason was that one child had more health issues than the other, that one stayed in the NICU for longer, or that one reminded them of their ex-partner or themselves more.
Wertz stated that cognitive stimulation, which is more beneficial for children who receive it more often, and discipline, which is more severe, seem to cause dissimilarities in identical-siblings.
This is true for identical twins. You can see how siblings with different geneticities could have their interests, accomplishments and personalities shaped by their environment, until they no longer look like each other.
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.
Parents should not only be concerned about sibling differences, but should also encourage them to do so. Wertz, a mother of two young daughters, stated that while you want to provide warmth, discipline, and various opportunities for cognitive stimulation to all your children, you also want to allow them to be themselves.
“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.”
