It is obvious to many of us – whether we are an only child or not – that siblings are often different from each other.

Judy Dunn, Emeritus Professor of Developmental Psychology at Kings College London, and Robert Plomin, Professor of Behavioral Genetics at the same institution, were among the first scientists to begin empirically asking why this happens. Drawing on the differences they noticed in the Dunn children, over the past 30 years they have tested the siblings’ variations in character and personality.

Finally, they — and other experts in the field — appear to have come close to answering why even identical twins who live under the same roof can sometimes turn out to be completely different in character.

How genetically similar are siblings?

Genetics usually predict how siblings will be different. “Because you and your brother are 50 percent genetically similar, that means you’re also 50 percent genetically different,” says Plomin.

If it were that easy, siblings would be half different and half the same. But genetics is not so linear. Not all genes are passed down equally, as they experience some shifting throughout the playback process.

Read more: Identical twins: how alike are they genetically?

Why do siblings look different?

And not many traits are predicted by just one gene: Even a trait as simple as eye color is dictated by a combination of genes working together. While eye color is actually a the result of about 16 different genes mixed, personality traits are determined by an even greater number of genes, meticulously linked together in a variety of combinations and formulas. Change even just one gene and the result will be completely different. So personality is less heritable than other traits.

“The more heritable a trait, such as height, the smaller the sibling differences on average. Weight is again surprisingly heritable,” says Plomin. “But siblings are somewhat different when you get to cognitive abilities, which are less heritable than weight and height, and even less similar when you get to personality and psychopathology.”

Where does all this difference come from?

Genetics help explain the similarities between siblings, but not so much the differences. Environmental factors may explain the rest. Children who grow up with the same two parents who attend the same schools can potentially experience vastly different environments, both subjectively and objectively.

“In families, the environment works differently than we thought, because it makes children in one family different from one another. It’s actually a non-shared environment,” says Plomin. Not to mention the differences that naturally arise from extramarital relationships: a teacher or a lover can completely change someone’s life.

What about parental favoritism?

The attitude of parents towards children can also affect their character. A team led by Susan Marie McHale, a professor of human development and family studies at Penn State University, argued that from the very beginning, two siblings cannot outlive their parents in the same phase of their livesbe it romantic, financial or personal.

As a first or second child, perspectives can easily change and research shows that this is not systematic. Not all older siblings have certain traits – they just have different experiences because they are older.

Differences in children’s personalities cause differences in the personalities of the people who care for them. “We find that when parents have mixed-gender children, such as a boy and a girl, they are more likely to believe that their children have a greater influence on how they are treated,” says McHale. The same goes for raising teenagers versus children, she says.

After all, parents also have preconceived notions and beliefs about their children and treat them a little differently – even if all parents say they absolutely don’t.

Read more: The secret reason parents play favorites

Self-fulfilling prophecy and grades

McHale’s published research in Journal of Family Psychology, for example, suggests that when parents perceive one child to be more academically gifted than the other, that child’s grades improve more than their siblings. However, this was not the other way around. Sometimes it almost becomes a self-fulfilling prophecy where small differences between siblings are exaggerated by their parents.

In some cases, it can help understand sibling differences, Plomin says. But statistically, these differences do not appear to be a sufficient link between all parents’ behavior and the personality of all their children.

In his book, Blueprint: How DNA Makes Us Who We Are, Plomin still suggests that although parents have a great deal to do with their children’s character, due to genetics, children are still predisposed to have certain personalities and abilities regardless. Parents’ job is to encourage their children to become the best version of themselves, he says.

Do siblings influence each other?

There is much to understand in the way siblings influence one another. “In a sibling relationship, you have different roles and different experiences as the giver versus the recipient of interactions and observations,” McHale says.

In some cases, siblings seem to be important influence on how adolescents act in risky situations, such as how much they drink and whether they use substances. In other cases, they mutually subtract their opposites. “Your sibling can be a role model or they can be a foil,” McHale says.

Competition – and even jealousy – between two siblings can reveal similar or different personalities. It’s along the lines of Darwin principle of divergence.

Everything can take into account

Since none of these theories seemed to yield satisfactory results, after 30 years of trying to identify systematic sources of sibling differences, Plomin argued that the question needed to be reformulated entirely.

“Part of the problem is asking, what is the cause of something?” Plomin says. “The answer is that these are complex developmental phenomena. The answer is probably a thousand things and different things to different people.

And chance and random experiences play a much bigger role in our lives than people tend to believe. “I think these idiosyncratic, stochastic, or random differences are a major reason why siblings are so different,” says Plomin.

Read more: Picky eaters: can we blame genetics?

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