Beauty, aggression, nervousness, depression, intelligence and the ability to make up limericks all run in families. But if you cannot find the genes underlining a personality trait, how can you possibly find out if it is inherited rather than learnt? This formidable question has a surprisingly simple answer: studied twins and study adopted children.
Identical twin is genetically identical. They always have, for instance the same color eyes. Fraternal twins have, on average, half their genes in common. Sometimes one twin is green eyed and the other blue eyed. They are so more alike genetically them any other two siblings. When identical twins are more similar for some trait then fraternal twins, we say that the trait is heritable.
This is true for eye color, but how about more complex traits, like limerick composing? Even if identical twins have the same limerick composing ability and fraternal twins are not as similar, this talent could still be the result of child rearing. Every one knows that identical twins are raised more similarly: their parents dress them the same, they share the same bedroom more often, they take the same classes and so on. Identical twins reared apart have the same genes, but they grow up in vastly different environments. If they are similar for some personality traits, it must be heritable and not learned. The study of identical twins reared apart is the best way to untangle the effects child rearing from the effects of genetics.
Indeed, if you want to attach a number for degree of heritability, simply take the correlation for identical twins reared apart: when that correlation 1.00, the trait is completely determined genetically; when it is lower say 0.50, this means that the trait is half genetic and half non genetic in origin.
