Studies of moral judgement often involve scenarios that participants rate for immorality and harmfulness. One such scenario describes consensual sex between two siblings named Mark and Julie. Many participants condemn the siblings’ behavior, even though the scenario was “carefully written to be harmless, by specifying that Mark and Julie used contraceptives, kept it a secret, and had a great time” (Gray & Pratt, 2025).

But if the siblings’ consensual sex was described as harmless, why did study participants condemn it? Because they believed it was harmful despite the description. And the perception of harm was immediate and intuitive, not some post-hoc rationalization of moral discomfort.

Gray and Pruitt maintain that perception of harm is central to all moral judgments. Or as they put it, “harmless wrongs do not exist”. They also argue that “moral disagreement across politics is in part grounded in different assumptions of vulnerability”. For example:

“Liberals are more likely than conservatives to view undocumented immigrants, gay people, and the environment as vulnerable to harm. Conversely, conservatives are more likely than liberals to view state troopers, the American flag, and the Bible as vulnerable.”*

Why would it matter that the American flag and Bible are “vulnerable”? Because they are objects that represent sacred commitments, such as commitment to country and Christianity. Much like wedding rings represent marital commitment. Small things speak to bigger things. And these particular commitments are all vulnerable, because they are threatened by cultural change.

So what sacred commitments might sibling incest jeopardize? For starters, the lifelong commitments of family and community.

But “jeopardizing lifelong commitments” sounds too cognitively complex to trigger an immediate and intuitive perception of harm. What’s needed for a snap moral judgment is some gut feeling or premonition that an act could lead to harm - a harm that any reasonably good person would want to avoid. In the case of incestuous siblings, that harm would likely unfold over time, set in motion by the power of sex to trigger unruly emotions and bad behaviors that ultimately pull the family apart. It might feel good right now but just wait.

In other words, don’t play with fire because it could burn down the house. And if you play with fire, expect to be condemned - even if you were ignorant of the risk. Because you should have known.**

Instead of sibling incest, why didn’t researchers use other, more plausible scenarios, such as the “cheating spouse” or the “boss/intern hookup”. Probably because carefully writing these scenarios to be harmless would just be too hard.  

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* Sounds like a cartoonish version of conservatives, who are quite a diverse lot. See Pew Research Political Typology Reports for numerous examples of the range of political opinion within conservative groups.

** Yes, the “should have known” is packed with assumptions, e.g., knowledge of moral risk requires sensitivity to moral risk, so ignorance of risk reflects insensitivity to the possibility of causing harm – a moral failing.

Reference

Gray, Kurt, and Samuel Pratt. "Morality in Our Mind and Across Cultures and Politics." Annual Review of Psychology 76 (2025).  https://doi.org/10.1146/annurev-psych-020924-124236