Parents develop long-term disgust habituation, but only after beginning to wean their children

Disgust helps humans avoid potentially pathogenic substances such as bodily effluvia. This reduces illness risks, and is difficult to overcome with cognitive strategies or through short-term habituation (minutes to hours). Whether long-term habituation (months to years) exists is an unsolved question. While regular professional exposure to disgust elicitors is associated with lower disgust sensitivity and avoidance, this could reflect selection and survivorship bias. Here, we use the natural quasi-experiment of parenthood: it greatly increases exposure to bodily effluvia, but disgust does not usually inspire individuals to start or stop being a parent. Parents (N=99) and controls (N=50) completed self-report and behavioural avoidance measures. We used parent-specific items in disgust-sensitivity questionnaires, and child-related stimuli (soiled diapers) in a preferential-looking task. These included diapers with pre-weaning (yellow and runny) or post-weaning faeces (adult-like). While the control group showed the expected behavioural avoidance, parents of weaning or weaned children showed almost no avoidance of stimuli depicting child-related or general bodily effluvia. Curiously, parents of pre-weaning children showed similar disgust avoidance to the control group, even if they had older children. These results suggest that parents habituated to disgust induced by faeces in diapers, and that this had generalised to other bodily effluvia. Contrary to our expectations, parents did show disgust avoidance while their (youngest) children were fed only milk, which could point to an adaptive response to reduce the risk of illness in young infants. In sum, after the sensitive milk-feeding stage, continuous exposure to their children’s bodily effluvia inoculates parents to disgust.