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Cass R. Sunstein, Discerning Blue from Purple: How Prevalence Affects What is Perceived as Normal, 44 Evolution & Hum. Behav. 250 (2023).

Abstract: How do judgments about law and morality shift? Why do we come to see political or other conduct as acceptable, when we had formerly seen it as unacceptable, immoral, or even horrific? Why do shifts occur in the opposite direction? Why accounts for the power of “the normal”? A clue comes from the fact that some of our judgments are unstable, in the sense that they are an artifact of, or endogenous to, what else we see. This is true of sensory perception: Whether an object counts as blue or purple depends on what other objects surround it. It is also true for ethical judgments: Whether conduct counts as unethical depends on what other conduct is on people's viewscreens. There are plausible evolutionary explanations for these findings. It follows that conduct that was formerly seen as unethical may come to seem ethical, as terrible behavior becomes more common, and also that conduct that was formerly seen as ethical may come to seem unethical, as good behavior becomes more common. In these circumstances, law (and enforcement practices) can have an important signaling effect, giving people a sense of what is normal and what is not.