Parents aren’t easily disgusted, but only once their child has started eating solids. The level of disgust that parents experience seems to change over time, which could have evolved to both protect their child and prime their immune system.

Disgust probably evolved as a way to avoid pathogens, such as those in faeces or vomit, as a sort of “behavioural immune system”, says Hannah Berg at the Laureate Institute for Brain Research in Tulsa, Oklahoma, who wasn’t involved in the new research.