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Understanding Sibling Personalities

Why are my kids so different?

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Sibling Differences
Source: The Light Photography/Shutterstock

Many parents assume that providing the same home environment for each of their children, will yield “similar” children, but decades of twin studies and recent findings from behavioral genetics suggest that differences in personality have substantial genetic underpinnings. The stage for who we become as adults is at least partially influenced by what genetics we inherit.

The Big Five Personality Characteristics?

Personality describes the relatively stable individual characteristics of thinking, feeling, and behaving that are generally consistent over time and situations. While it is entertaining to determine our Hogwarts house via an online quiz or to answer a survey in a grocery store magazine, most personality inventories available to us are not theoretically sound. They are neither reliable nor valid.

One exception to these underwhelming personality tests is the Five-Factor Model of Personality:

  1. Conscientiousness
  2. Agreeableness
  3. Neuroticism
  4. Openness
  5. Extraversion

These five characteristics seem to be reliable and valid over time and across cultures. For this reason, it is intriguing to study the Big Five and determine to what extent our genetics versus our environment influence these psychological characteristics.

Twin Studies

Explaining the basics of how psychologists have studied the nature versus nurture question for over a century is helpful to all parents seeking understanding of sibling personality disparities. So here are some basic concepts:

  1. Monozygotic (identical) twins develop from a single fertilized egg that split in two. They share 100% of the same genetics.
  2. Dizygotic (fraternal) twins develop from two separate fertilized eggs. They share (on average) 50% of the same genetics, just like typical biological siblings.

In the late 1970s, when many psychologists believed that environmental influences were the primary driver in personality and development, Thomas Bouchard and his colleagues at the University of Minnesota began studying identical twins who were reared apart. This study spanned over two decades and provided a wealth of information regarding nature versus nurture in regards to many different physical and psychological variables. Many fascinating stories came about regarding identical twins reared apart who met for the first time in mid-life. One of the most famous was the study of the Jim Twins.

Concordance and Heritability

Within twin research, concordance is a categorical measure that determines to what extent members of a twin pair have a particular psychological trait, such as schizophrenia or depression. Bouchard’s study indicated substantial concordance rates for identical twins reared apart on many psychological variables. Of interest, concordance rates for identical twins reared apart were similar to those of identical twins raised together, suggesting the environmental influence was based on unshared environmental variables.

Heritability, on the other hand, is a quantitative measure of the correlation between twins’ scores on tests of psychological characteristics. The Big Five personality dimensions show about 40-60 percent heritability rates, indicating that a sizeable portion of personality appears to be determined by genetic factors.

Behavioral Genetics

Human behavioral genetics directly investigates how our underlying genetics influence our individual differences in behavior. This area of study is complex and beyond the scope of this article, but the field is quickly progressing.

Like results from twin research, studies in the field of behavioral genetics suggest that differences in the genes we inherit seem to matter on many psychological variables, including personality. However, for the vast majority of issues related to human psychology, it isn’t as simple as one gene accounting for one psychological characteristic. Research is suggesting that many genes often contribute to the overall polygenic variance that shapes behavior. See Robert Plomin’s book, Blueprint, for an in-depth discussion of the rapidly progressing field of behavioral genetics.

Beyond the genes we inherit, an essential factor for parents to consider is what Plomin calls the “nature of nurture.” By this, he means the underlying genetics work to elicit responses from parents and others, in addition to children creating their own environments based on their genetic propensities. As parents, we can probably see how this could happen. One child is easy and doesn’t demand attention, while another is challenging. Our reactions (and other people’s reactions) to them will be different, therefore, creating different environments. Also, for example, one child may be musically inclined and bother parents until they pay for piano lessons. Plomin articulated this phenomenon in his recent article.

Conclusion and Implications

In 1930, behavioral psychologist John Watson famously said, “Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I’ll guarantee to take anyone at random and train him to become any type of specialist I might select—doctor, lawyer, artist, merchant-chief and, yes, even beggar-man and thief, regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors.”

Over the past several decades, science has shown us that Watson’s thinking was flawed. Biology matters. It is not destiny, but it significantly influences our development. The environment, of course, also matters a great deal. Environmental deprivation and or abuse can have harmful effects on children as they develop.

Parenting also matters. As parents, however, we need to keep in mind that children, as Robert Plomin stated, are “not blobs of clay” to be molded into whatever it is that we desire. We can guide and provide experiences, but we likely can’t change their fundamental personalities.

On the one hand, this is challenging to accept as a parent, especially if a child is vastly different than other family members (e.g., an introvert in a family of extroverts, or vice versa). On the other hand, it is freeing for parents. We should sit back and let our children unfold, with a bit of guidance, and love them for who they are.

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