Summary
Highlights
Dr. Nancy Segal, a psychology professor and fraternal twin, explains her initial curiosity about twin differences, leading her to study genetics. She spent years at the Minnesota Study of Twins Raised Apart, comparing identical and fraternal twins.
Identical twins share all their genes, originating from a single fertilized egg, while fraternal twins share 50% of their genes, like ordinary siblings, resulting from two separate eggs and sperm. Comparing similarities between these twin types helps determine the influence of genetics.
Twin studies reveal significant genetic components in many behaviors, more than previously thought. Physical traits like height, weight, and general intelligence have substantial genetic influences. Surprisingly, traits such as religiosity and political attitudes also show genetic components.
Genes predispose but do not determine outcomes. Their expression can change based on the environment. Even identical twins with the same DNA can have different gene expressions due to environmental differences, potentially starting in the womb.
Identical twins raised apart share genes but not environments, making any resemblances attributable to genetics. Dr. Segal highlights astonishing similarities found in these twins, not just in intelligence and personality, but also in unusual habits and specific physical symptoms, suggesting a strong genetic link interacting with environments.
Dr. Segal emphasizes that while genes predispose, they are not deterministic, meaning individuals are not 'set in stone'. Environments can be altered to change behavioral expression, prevent or mitigate diseases. This understanding allows for intervention while acknowledging that not everyone can be the same.