This article claims that our natural disgust at parasites and other gross things is the origin of morality.

A ballooning body of research by Pizarro and others shows that moral judgments are not always the product of careful deliberation. Sometimes we feel an action is wrong even if we can’t point to an injured party. We make snap decisions and then – in the words of Jonathan Haidt, a social psychologist at New York University – ‘construct post-hoc justifications for those feelings’. This intuition, converging lines of research reveal, is informed by disgust, an emotion that most scientists believe evolved to keep us safe from parasites. Marked by cries of ‘Yuck!’ and ‘Ew!’, disgust makes us recoil in horror from faeces, bed bugs, leeches and anything else that might sicken us. Yet sometime deep in our past the same feeling that makes us cringe at touching a dead animal or gag at a rancid odour became embroiled in our most deeply held convictions – from ethics and religious values to political views…

These and related studies raise an obvious question: how have parasites managed to insinuate themselves into our moral code? The wiring scheme of the brain, some scientists believe, holds the key to this mystery. Visceral disgust – that part of you that wants to scream ‘Yuck!’ when you see an overflowing toilet or think about eating cockroaches – typically engages the anterior insula, an ancient part of the brain that governs the vomiting response. Yet the very same part of the brain also fires up in revulsion when subjects are outraged by the cruel or unjust treatment of others. That’s not to say that visceral and moral disgust perfectly overlap in the brain, but they use enough of the same circuitry that the feelings they evoke may sometimes bleed together, warping judgment…

From this point in human social development, it took a bit more rejiggering of the same circuitry to bring our species to a momentous place: we became disgusted by people who behaved immorally. This development, Curtis argues, is central to understanding how we became an extraordinarily social and cooperative species, capable of putting our minds together to solve problems, create new inventions, exploit natural resources with unprecedented efficiency and, ultimately, lay the foundations for civilisation.

This has interesting implications for the idea that there might be a “common morality”, akin to “common sense”, that just naturally applies to all rational people, although rational people might disagree about exactly what is included, just as scientists argue about the nature of reality but agree there is a single reality that can eventually be discovered. This is powerful because we can’t rely on reason alone as a guide to morality – there are sometimes things we could do that would be rational, but almost everyone would agree are wrong. Obvious examples would be if you could benefit yourself by lying, cheating, stealing, or killing, and be sure you could get away with it. In this case your gut tells you this is wrong even though it might be strictly rational, and that will be enough to deter most people. But if that gut sense of common morality is based partly on biological impulses shaped by past conditions that no longer apply, then maybe we should rely more on reason and less on our gut impulses of what is right and wrong.