In most mammalian brains, the insula’s function is straightforward. If a rat, say, bites into rancid food, its insula protects it from being poisoned by triggering nausea and gagging. In humans, however, the insula is not merely about gustatory disgust. If someone recounts something “rotten” they once did, or hears about someone else’s similar behavior, their insula activates. In other words, the insula also processes moral disgust. For example, if someone backstabs you in a game, the magnitude of your insula’s activation predicts how outraged you’ll feel, and how vengeful you’ll act. Our insula responds mainly to the disgustingness of sentient, intentional harm—if the person stabbed in the back believes a computer was to blame, her insula remains quiet. And if we’re sufficiently morally disgusted, we even feel sick to our stomachs. Our brains fail to distinguish between literal disgust at a fetid taste and metaphorical disgust at a rotten act. I touch on this neurobiological quirk in my latest book, Behave.
This intertwining of the literal and metaphorical reflects evolutionary tinkering, not inventing. When hominins first developed this capacity for morality, it wasn’t driven by a new brain region. Instead, evolution duct-taped moral disgust onto the insula’s existing repertoire, and the region occasionally confuses it with gustatory disgust.
This intertwining certainly has an upside. Righting moral wrongs can demand great sacrifice, and it would be hard to work up a head of steam for that if moral transgressions were mere detached abstractions. A stomach churning with outrage can supply the visceral fuel that makes moral imperatives feverishly vital.
But there are downsides. It’s tempting to make disgust a litmus test for deciding what’s wrong—if it makes you puke, then you must rebuke. But one person’s repugnance is another’s loving lifestyle. Moral disgust is mired in time, place, and parochialism.
A stomach churning with moral outrage can also invite hatred, equating “different” with “deplorable,” deciding that They think, live, love, and pray abnormally.
All despots know this, and rely on activating their minions’ insulas by making the objects of their personal outrage into something more broadly disgusting: drug smugglers and rapists; blasphemers and infidels; vermin and malignancies.
There’s another danger. It’s hard to meet a stranger and know if they share your deepest values. But if those values are yoked to a marker, such as style of dress, the task becomes easier. Know nothing more about two people than that one wears a Stetson and the other a sari, and you can confidently guess who of the two eats cows versus worships them.
This provides a dangerous stepping-stone. Being disgusted by someone’s abstract beliefs is tough for an insula. But feeling disgusted by others because, say, they eat repulsive, sacred, or adorable animals—this the insula can sink its teeth into. This impulse supplies the momentum to decide that, as long as you’re at it, They also have disgusting ideas about, say, deontological ethics.
If a subordinate baboon is brutalized by a dominant thug, a third baboon may be galvanized into protecting the victim (especially if they are related). But only a human would fight the unjust treatment of a distant stranger, an endangered species, or a fragile ecosystem. My guess is that this only occurs because of the human insula’s uniqueness: the fact that an injustice can make our innards blaze with a need to act. But that same blazing leads so many to grab pitchforks and join the mob. Innovation can be cool—like controlling fire, Snapchat filters, or the insula becoming a multitasker. But innovation usually turns out to be a double-edged sword.
Robert Sapolsky is a neuroendocrinologist at Stanford University and the author of several books. Read an excerpt of Behave: The Biology of Humans at Our Best and Worst.