A mechanism to increase argument strength with negative evidence

A mechanism to increase argument strength with negative evidence Daniel Heussen (Daniel.Heussen@psy.kuleuven.be) Wouter Voorspoels (Wouter.Voorspoels@psy.kuleuven.be) Gert Storms (Gert.Storms@psy.kuleuven.be) Department of Psychology, University of Leuven Tiensestraat 102, 3000 Leuven, Belgium Abstract In most cases, if relevant, positive evidence raises and negative evidence lowers argument strength in induction. Previous research, however, has shown that it is possible to raise the argument strength of a single positive premise argument by introducing negative evidence (Heussen, et al., 2011). Here we test one possible mechanism for such an increase in argument strength. When people consider the positive premise they develop a set of hypotheses. Subsequently encountering a negative premise would render some of these hypotheses less likely and hence, if participants see the hypotheses as an exhaustive set, shift the probability to the remaining hypotheses to varying degrees. We test this idea by asking people to choose between four hierarchically nested conclusions across various conditions of evidence. The results are discussed in the light of models of induction. Keywords: Induction; Negative evidence; Hypothesis Space Negative evidence in induction In its broadest sense, inductive reasoning can be defined as inference to an uncertain conclusion (Lipton, 2004). There are at least two main approaches to make such inferences. One is to generalize over time. “My car has always started, so it’s reliable and will start today. ” There are a number of philosophical arguments why such an approach is not reliable (Russell, 1912), but owning an aging car will quickly teach you the same. Another approach is to generalize across instances of a category. “My car is a German car, so it’s reliable and will start today. ” Although this approach similarly has its problems, it is certainly pervasive in everyday reasoning. As a consequence a large amount of research, particularly in psychology, has been looking at category-based property induction (see Feeney & Heit, 2007 and Heit, 2000 for overviews). In these studies, participants might be asked to judge how likely it is that Bobcats use serotonin as neurotransmitter given that both Tigers and Cougars do (Smith, Shafir & Osherson, 1993). What is striking though is that in the majority of this research the focus has been on positive evidence, evidence that states that some entity has a particular property. And only very little is known about the influence of negative evidence, evidence that states that some entity does not have a particular property. So what do we know about the influence of negative evidence on argument strength? In work on category-based property induction, negative evidence has been found to behave in a similar manner to positive evidence, however, with the reverse effect on argument strength (Heussen & Hampton, 2009; Heussen, Voorspoels, & Storms, 2010). For instance, just as for positive evidence the similarity between the negative evidence and the conclusion is a key predictor of the influence of the evidence on the conclusion. Similarly, in generalizations to the category, negative evidence instantiated by a less typical exemplar has a less detrimental impact on argument strength than from a more typical exemplar. Other studies have shown that in mixed premise arguments, containing both positive and negative evidence, the similarity between the contradicting evidence plays an important role with greater similarity between positive and negative evidence resulting in a stronger impact of the negative evidence (Blok, Medin & Osherson, 2007). In line with intuition, these studies show that positive evidence raises argument strength—“Lions have enzyme x, tigers have enzyme x, how likely is it that cheetahs have enzyme x? ”—and negative evidence lowers it—“Lions have enzyme x, tigers DO NOT have enzyme x, how likely is it that cheetahs have enzyme x? ” Argument strength never goes against the “sign” of the evidence. Elsewhere we call this the Monotonicity Principle (Heussen et al., 2011). Against the monotonicity principle Contrary to the Monotonicity Principle, however, it has been found that in some circumstances it is possible to raise argument strength by introducing negative evidence (Heussen et al., 2011). Participants were asked to consider scenarios like the following: “Scientists have established that in certain brain regions the music of Dmitri Shostakovich elicits particular brain waves called alpha waves. Given that Shostakovich’s music causes these alpha waves, you might wonder whether this similarly applies to other kinds of music like, for instance, the music of Bach. ” Participants were then asked to judge how likely it is that Bach’s music also causes alpha waves? Participants then received additional information, that the same group of scientists found that the music of the hard rock band AC/DC does NOT elicit alpha waves in the brain. And again they