On facts and conceptual systems: young children's integration of their understandings of germs and contagion.

Five studies argue against claims that preschoolers understand a biological germ theory of illness. In Studies 1-3, participants were read stories in which characters develop symptoms (e.g., a bellyache) caused by germs, poisons, or events (e.g., eating too much candy) and were asked whether another character could catch the symptoms from the first. Few children made judgments in terms of germs as part of an underlying causal process linking the origin of a symptom to its subsequent transmission. Some children may have reasoned simply that certain kinds of symptoms are likely to be contagious. Studies 4 and 5 undermined the claim that preschoolers understand germs to be uniquely biological causal agents. Young children did not attribute properties to germs as they did for animate beings or for plants. It is suggested that children undergo conceptual reorganization in constructing a Western adult understanding of germs.