On just about every trait that has been studied, identical twins (who share all their genes and spend the same nine months in the same womb) are more similar than same-sex fraternal twins (who share only half their genes and spend the same nine months in the same womb). This finding means that genes make at least some contribution to nearly every trait. Whether the trait is intelligence, extroversion, fearfulness, religiosity, political leaning, liking for jazz, or dislike of spicy foods, identical twins are more similar than fraternal twins, and they are usually almost as similar if they were
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