![image](belsky4e_fig01_un13.jpg)
How “genetic” are these children’s friendly personalities? To answer this question, researchers compare identical twins, such as these two girls (above), with fraternal twins, like this girl and boy (below). If the identicals (who share exactly the same DNA) are much more similar to each other than the fraternals in their scores on friendliness tests, friendliness is defined as a highly heritable trait.
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n-Francis Bourke/Getty Images