Since the subject of twins has arisen, I want to bring up the topic of heritability calculations. We know of the myriad problems we have in interpreting these and there might be one more. MZ twins are always of the same sex. DZ twins may or may not be. Yet many textbooks claim that the higher concordance rates for personality traits, psychological illnesses, etc (which are often gender related) for MZ vs DZ indicates genetic effects. Would not a better comparison for heritability be MZ twins vs DZ twins of only the same gender.
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