Despite sharing genes and, in most cases, environment, siblings are can be very different. Since the 1980s, U.S. psychologist Robert Plomin has carried out several studies into behavior genetics, sibling differences and twins. The term "non-shared environment" was coined by Plomin to describe the environmental reasons siblings brought up in the same home are often so different. According to Plomin, siblings tend to have similarities in physical appearance and cognitive abilities, but when it comes to personality, everyone knows how different they can be.
Siblings can look alike, but sometimes they do not resemble each other at all. Physical differences come down to two factors: environment and genetics. Geneticists use the term environment to describe the manner in which a person grows up. This includes location, diet and lifestyle. Part of the reason siblings look different is that no two people grow up in exactly the same conditions, even twins. Although siblings have the same parents, they don't have the same genes, which influences significant aspects of a person's appearance, from hair color and eye color to whether he has freckles or not. Everybody has two versions of most of their genes and parents pass one version of each of their genes to their children, completely randomly.
A child's cognitive development might be affected by several factors, including home environment and genetics. Siblings who are brought up in the same home by the same parents might not, however, be similar in terms of cognitive abilities, such as language skills, social skills, decision-making ability, memory and perception. This is partly because siblings experience the same circumstances at different stages of their own lives. For example, a 9-year-old child whose parents divorce is going to be affected a very different way than her 2-year-old brother. Scientists remain uncertain as to what extent cognition is affected by genetics.
Personality tests carried out by Plomin and other psychologists have concluded that siblings have similar personalities only around 20 percent of the time. While it is natural to assume that siblings who grow up in the same household, with the same parents, will turn out to be similar to one another, the opposite is true. According to Plomin, being raised in the same family actually makes children turn out to be quite different, personality-wise. Professionals remain unsure as to exactly why that's the case. The three main theories are divergence, environment and exaggeration. Divergence was first identified by Charles Darwin in the "Origins of Species;" Divergence exists to minimize direct competition as a survival strategy. In the case of siblings, for example, if one child excels at sport, his sibling will specialize -- whether they are aware of it or not -- in a different, noncompeting area such as art. Environment relates to Plomin's nonshared environment theory. Even if siblings grow up in the same family their own experiences will be different, partly because of age difference. Additionally, even if parents strive to treat each of their children in the same way, this rarely turns out to be the case. Exaggeration refers to the tendency of families to compare siblings and exaggerate even the smallest difference in personality. When specific labels are assigned to children, such as "the quiet one" or "the funny one," those small differences are reinforced and life choices are influenced accordingly.
A misconception about twins is that they are more alike than regular brothers and sisters. Fraternal -- nonidentical -- twins have around half of their genetic makeup in common, which is the same as any siblings. Identical twins, on the other hand, have the same set of genes. All the same, differences can occur even before identical twins are born. Even if twins share a placenta, one of them may be closer to the placenta than her sibling, meaning she receives nutrients earlier and turns out to be the larger twin both at birth and throughout their lifetimes. Twins often share a close bond and spend more time together than other brothers and sisters, but they can develop quite different personalities all the same.
- NPR: Siblings Share Genes, But Rarely Personalities
- Simine.com: Why Are Children in the Same Family So Different? Nonshared Environment a Decade Later; Robert Plomin, Kathryn Asbury and Judith Dunn
- The Tech Museum of Innovation: Relatedness
- The Society of Labor Economists: Like Brother, Like Sister? – The Importance of Family Background for Cognitive and Non-Cognitive Skills; Silke Anger and Daniel D. Schnitzlein
- Brain: The Genetic Basis of Cognition
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