Preschoolers' understanding of germs as invisible mechanisms

Recent studies have presented conflicting claims regarding whether young children's reasoning about biological content involves a unique set of causal mechanisms and theoretical entities. Three studies examined preschoolers' understanding of nonobservable causal mechanisms in causes of illness. According to traditional accounts, these children know that certain behaviors lead to illness but have no idea why or how. Many of the behaviors children cite as causes are actually mediated by the action of germs (e.g., contamination and contagion). Do children recognize that germs (nonobvious, invisible particles) are the mechanisms involved in some cases of illness causation? Study 1 demonstrates that 4- and 5-year-olds' predictions of who will get sick in cases of contamination and contagion are based on the presence or absence of germs. Study 2 serves as a control and further tests how children generalize this mechanism: Which causes do children think are mediated by germs? Data suggest that preschoolers understand but undergeneralize the role of germs. A final study indicates that younger preschoolers (3-year-olds) recognize that appearances may be deceiving when it comes to judging causes of illness. This understanding would seem to be a precursor to beliefs about specific mechanisms. Results are discussed in terms of commonsense theories and early conceptions of biology.