We are often guided by a gut sense of good and evil. But where does that feeling come from? Psychologist Jordan Theriault-Brown argues it has its roots in how a brain manages the uncertainty of the world around it, including the metabolic costs of that uncertainty. Moral feelings enforce rules that make our behavior predictable to ourselves and others. The social and psychological dynamics that follow from this energy-saving trick can motivate violence as well as bravery. This idea can explain behavior that evolutionary accounts struggle with, such as extreme self-sacrifice. When someone dies for a cause, Theriault-Brown suggests, it’s because abandoning their most foundational moral principles would force their brain to rebuild its entire model of who they are.

Why would someone feel a duty to die for a cause? Think of Joan of Arc on the pyre, soldiers charging into machine gun fire, or even terrorists in a suicide attack. Morality is often thought to be the angel on our shoulder, motivating us to do good, but in reality, morality is much more complicated. It can motivate great acts of bravery, but it can also motivate horrendous acts of violence, all in the name of what is felt to be “right.”

Where does this feeling of duty come from, and how might it even motivate self-sacrifice? This question is particularly puzzling from an evolutionary perspective: if our behaviors are “programmed” into us by our genes, and if those genes were selected to promote survival, then how could someone find themselves feeling bound by duty to die for a cause? The usual evolutionary answers of kin selection and reciprocal altruism aren’t helpful here for obvious reasons (those charging soldiers aren’t related, and Joan of Arc won’t be around to receive favors in turn). Likewise, reputation-based explanations aren’t fully satisfying either, and generally assume you’ll be around to reap the benefits of your actions. One could invoke belief in God and rewards in the afterlife to explain it, but nonreligious people also engage in moral self-sacrifice—for example, protestors have risked their lives to do what they feel is right, and doctors have died to give aid in war zones. Was every protestor and doctor compelled by rewards from God, or by some reputational or evolutionary self-interest? Or, as Kant proposed, might moral duty be something recognizable only when it runs counter to our preservation, welfare, and happiness? If we’re willing to grant the psychological fact that we can, at times, feel compelled by duty (deontology means “the science of duty”), then how could that feeling of duty emerge from the meat of a brain?

To answer this, we need to first take a step back from morality and consider the broader category of norms.

While morality concerns all the more dramatic stuff—good, evil, life, death, and so on—norms refer to the more common kinds of behaviors expected of us in the day to day. Social pressure pushes us to conform to these norms, which are really just the collective expectations of others. This normative social pressure has a certain feeling to it: you don’t just want to conform, or choose to conform, but rather, you feel like you really should conform. This isn’t the full-blown duty of moral obligation yet, but violating norms does feel bad in a particular way—for example, think about the feeling of showing up to a Halloween party as the only one without a costume (or as the only one wearing one).

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It’s not others’ threats of punishment or ostracism that guide norm compliance; rather, it’s the simple avoidance of metabolic and affective discomfort.

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