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You typically do not need to
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because they do not want to have sex with one another.
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Now, this is--actually also illustrates
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the difference between proximate and ultimate causation.
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So, you think for yourself,
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"Eew. Do I want to have sex with my sister?"
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You don't think to yourself, "I would prefer not to,
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for the offspring that we will create will be nonviable
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and it'll be a waste of my reproductive efforts."
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Rather, you think, "Eew,"
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because at a gut level you respond.
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And this sort of instinctive response