Why (And When) Are Young Children So Bad at Avoiding Germs?
– by Serkan Özçakan, Florian van Leeuwen, and Joshua M. Tybur
Young children often behave as if they are indifferent to infection. They eat food from the floor, put dirty fingers in their mouths, and readily touch dirty surfaces. Importantly, young children do not just fail to take steps to avoid pathogens; they behave in ways that seem to approach pathogen risks. Paradoxically, all of this happens at a time when children are particularly vulnerable to pathogens, with their immune systems not yet mature. But why?
To answer this question, we first need to know when children begin avoiding pathogen threats and stop embracing them. The field currently lacks such a picture, for several reasons. Different relevant phenomena, including disgust, hygiene behaviors, and an understanding of contamination, have largely been studied separately. Measurement has sometimes been limited to questions asking parents to estimate the degree of disgust their child would experience during hypothetical events that the child may have never encountered. Child age is often binned into multi-year categories. Our study aimed to build a more comprehensive developmental map of pathogen avoidance by examining its multiple components simultaneously and at a high resolution of age.
To do so, we asked 3,050 parents in the UK and US whether they had observed each of 39 concrete behaviors in their child over the past month. Items concerned behaviors that increase pathogen exposure, such as eating food from the floor or touching dirty surfaces, as well as behaviors that reduce it, such as washing their hands or experiencing disgust. By recruiting parents of children from infancy to 10 years of age, we were able to estimate the prevalence of these behaviors across childhood in fine detail.
Our results showed that there was no single developmental timetable for pathogen avoidance. Instead, different behaviors emerged at different points in childhood. Disgust is a good example. Taste-related disgust and facial expressions of disgust toward food were already common by around six months. Vocalizations of “yuck” appeared around 20 months, followed soon after by more general disgust faces and reactions to sights, smells, and touch. The use of the word “disgust,” however, did not become common until around four years of age. In other words, disgust did not emerge all at once; its different components followed different developmental timelines.
We then asked whether behaviors with similar developmental patterns tended to group together. We observed four clusters of items with distinct developmental trajectories, which revealed two broader transitions around the second and fourth years of life. Around age two, a cluster of early exposure-increasing behaviors, including mouthing objects, became uncommon, while a cluster including assisted hygiene and early disgust responses became common. By age four, a second cluster of risk-prone behaviors, such as eating food from the floor, had also become uncommon, while more autonomous forms of pathogen avoidance, such as independent handwashing and toothbrushing, along with various expressions of disgust (e.g., saying “disgust” and reacting with disgust to things they saw or smelled), became common.
So, returning to the question we started with: why is pathogen avoidance so limited early in life despite high infection risk? Knowing when these behaviors develop gives us a starting point for thinking about why they follow this pattern. Many of the behaviors that are especially common early in life, such as mouthing inedible objects, licking dirty surfaces, and putting dirty hands in the mouth, allow children to explore their environment. This exploration may be important in at least two ways. It provides opportunities for learning about the world, but it also brings children into contact with microbes that may help shape the developing immune system.
The timing is particularly interesting here. The first 1,000 days of life are considered a key window for microbial colonization and immune development, when both the microbiota and the immune system are especially malleable (Zhu et al., 2025). Notably, this period broadly overlaps with the period in our data when behaviors that increase pathogen exposure were most common.
This overlap suggests that limited pathogen avoidance may reflect an adaptive trade-off between protection and exploration. High investments in pathogen avoidance from the beginning of life could limit opportunities for exploration at precisely the time when exploration may be especially important for learning and immune development. As children grow, this balance may shift toward protection. Behaviors that increase pathogen exposure become less common, while more organized forms of pathogen avoidance emerge. From this perspective, limited pathogen avoidance early in life may not simply reflect what young children cannot yet do. It may also reflect what they need to do: explore the world around them.
References
Zhu, J., He, M., Li, S., Lei, Y., Xiang, X., Guo, Z., & Wang, Q. (2025). Shaping oral and intestinal microbiota and the immune system during the first 1,000 days of life. Frontiers in Pediatrics, 13, Article 1471743. https://doi.org/10.3389/fped.2025.1471743
