BOX 5-2

The Interdependence of Theory and Evidence in Scientific Reasoning

In studies conducted by Koslowski and her colleagues, participants were given problem situations in which a story character is trying to determine if some target factor (e.g., a gasoline additive) is causally related to an effect (e.g., improved gas mileage). They were then shown either perfect covariation between the target factor and the effect or partial covariation (4 of 6 instances). Perfect correlation was rated as more likely to indicate causation than partial correlation. Participants were then told that a number of plausible mechanisms had been ruled out (e.g., the additive does not burn more efficiently, the additive does not burn more cleanly). When asked to rate again how likely it was that the additive is causally responsible for improved gas mileage, the ratings for both perfect and partial covariation were lower for all age groups.

Koslowski also tried to determine if participants would spontaneously generate information about causal mechanisms when it was not cued by the task. Children and adults were presented with story problems in which a character is trying to answer a question about, for example, whether parents staying in the hospital with them improves the recovery rate of their children. Participants were asked to describe whatever type of information might be useful for solving the problem. Half of the participants were told that experimental intervention (that is, parents and children could not be assigned to particular groups) was not possible, while the other half were not restricted in this manner. Almost all participants showed some concern for a causal mechanism, including expectations about how the target mechanism would operate. Although the sixth graders were less likely to generate a variety of alternative hypotheses, all age groups proposed appropriate contrastive tests.

relation between the illness and having recently cleaned carpets. Similarly, Thagard (1998a, 1998b) describes the case of researchers Warren and Marshall, who proposed that peptic ulcers could be caused by a bacterium, and their efforts to have their theory accepted by the medical community. The bacterial theory of ulcers was initially rejected as implausible, given the assumption that the stomach is too acidic to allow bacteria to survive.

Studies with both children and adults reveal links between reasoning about mechanism and the plausibility of that mechanism (Koslowski, 1996). When presented with an implausible covariation (e.g., improved gas mileage and color of car), participants rated the causal status of the implausible cause (color) before and after learning about a possible way that the cause could bring about the effect (improved gas mileage). In this example, par-



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