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Category-Based Induction Psychological Review Copyright 1990 by the American Psychological Association, Inc. 1990, Vol. 97, No. 2, 185-200 0033-295X/90/J00.75 Category-Based Induction Daniel N. Osherson Edward E. Smith Massachusetts Institute of Technology University of Michigan Ormond Wilkie Alejandro Lopez Massachusetts Institute of Technology University of Michigan Eldar Shafir Princeton University An argument is categorical if its premises and conclusion are of the form All members ofC have property F, where C is a natural category like FALCON or BIRD, and P remains the same across premises and conclusion. An example is Grizzly bears love onions. Therefore, all bears love onions. Such an argument is psychologically strong to the extent that belief in its premises engenders belief in its conclusion. A subclass of categorical arguments is examined, and the following hypothesis is advanced: The strength of a categorical argument increases with (a) the degree to which the premise categories are similar to the conclusion category and (b) the degree to which the premise categories are similar to members of the lowest level category that includes both the premise and the conclusion categories. A model based on this hypothesis accounts for 13 qualitative phenomena and the quanti- tative results of several experiments. The Problem of Argument Strength belief in the conclusion of an argument (independently of its premises) is not sufficient for argument strength. For this rea- Fundamental to human thought is the confirmation relation, son, Argument 1 is stronger than Argument 2 for most people, joining sentences P, ... Pn to another sentence C just in case even though the conclusion of Argument 2 is usually considered belief in the former leads to belief in the latter. Theories of con- more probable than that of Argument 1. An extended discus- firmation may be cast in the terminology of argument strength, sion of the concept of argument strength is provided in Osher- because P\ ...P, confirm C only to the extent that />,... Pnf son, Smith, and Shafir (1986). It will be convenient to qualify C is a strong argument. We here advance a partial theory of an argument as strong, without reference to a particular person argument strength, hence of confirmation. S, whenever the argument is strong for most people in a target To begin, it will be useful to review the terminology of argu- population (e.g., American college students). We also say that ment strength. By an argument is meant a finite list of sen- P,... P, confirm C if P, ... PJC is strong. tences, the last of which is called the conclusion and the others An illuminating characterization of argument strength its premises. Schematic arguments are written in the form P, would represent a long step toward a theory of belief fixation ... PJC, whereas real arguments are written vertically, as in and revision. Unfortunately, no general theory is yet in sight, the following examples: and even partial theories are often open to elementary counter- Grizzly bears love onions. examples (see Osherson et al., 1986). This article offers a hy- Polar bears love onions. pothesis about the strength of a restricted set of arguments, ex- All bears love onions. (1) emplified by Arguments 1 and 2. The premises and conclusions of such arguments attribute a fixed property (e.g., preys on Owls prey on small rodents. (2) small rodents) to one or more categories (e.g., OWL and RATTLE- Rattlesnakes prey on small rodents. SNAKE).' The present study focuses on the role of categories in An argument A is said to be strong for a person 5 just in case 5s confirmation; the role of properties is not systematically investi- believing A's premises causes S to believe A's conclusion. Mere gated. In this sense, the model we advance concerns induction that is category based. Category-based induction was first examined by Rips (1975). He studied the strength of single-premise arguments involving Support for this research was provided by National Science Founda- categories such as RABBIT and MOUSE, or EAGLE and BLUEJAY. tion Grants 8609201 and 8705444 to Daniel N. Oshereon and Edward The present investigation builds on one of the models that Rips E. Smith, respectively. discusses and applies it to a larger class of arguments. We thank Lawrence Barsalou, Lee Brooks, Susan Gelman, Ellen Our discussion proceeds as follows. After defining the class Markman, Douglas Medin, Lance Rips, Joshua Stern, and an anony- of arguments to be considered in this article, and introducing mous reviewer for helpful discussion. some relevant terminology, we document a set of 13 qualitative Correspondence concerning this article should be addressed to Ed- ward E. Smith, Department of Psychology, University of Michigan, 330 Packard Road, Arm Arbor, Michigan 48104. ' We use capitals to denote categories. Properties are italicized. I8S 186 OSHERSON, SMITH, WILKIE, LOPEZ, SHAFIR phenomena that must be deduced by any adequate theory of ise P or conclusion C, we denote the category that figures in P argument strength. Our own theory is then presented and or C by CAT(.P) or CAT(C). Thus, if P is the first premise of shown to account for all of the phenomena. Next, we describe Argument 1 above, then CAT(P) = GRIZZLY BEAR. If C is the several experiments designed to test the theory quantitatively. conclusion of Argument 3, then CAT(C) = BEE. Refinements and alternatives to the theory are discussed in the Let Argument A = P, ... Pn/C be given. A is called general final section. if CAT(.PI) ... CATCPn) are all properly included in CAT(C). For example, Argument 1 is general. A is called specific if any cate- Arguments To Be Considered gory that properly includes one of CAT(T'i)... CAT(Pn), CAT(C) also properly includes the others. For example, Argument 3 is An argument is called categorical just in case its premises specific. By this definition, no argument is both general and spe- and conclusion have the logical form all members of X have cific. A is called mixed if A is neither general nor specific. The property Y, where X is a (psychologically) simple category like following argument is mixed: FALCON, VEHICLE, or MAMMAL, and Y remains fixed across Flamingoes require titanium for normal muscle premises and conclusion. Arguments 1 and 2 are categorical in development. this sense. The arguments discussed in this article are all cate- Mice require titanium for normal muscle development. gorical. All mammals require titanium for normal muscle (4) The property ascribed to the categories figuring in Argument development. 1 is loves onions. Subjects are likely to have prior beliefs about the kinds of animals that love onions, as well as prior beliefs Argument 4 is not general because FLAMINGO is not included about properties that are correlated with this one, such as eats in MAMMAL. It is not specific because BIRD properly includes a wide variety of fruits and vegetables. Beliefs such as these can FLAMINGO but not MOUSE or MAMMAL. Argument 2 is also be expected to weigh heavily on argument strength, defeating mixed. our goal of focusing on the role of categories in the transmission of belief from premises to conclusions. For this reason, the argu- Phenomena ments to be examined all involve predicates about which sub- jects in our experiments have few beliefs, such as requires biotin General Remarks for hemoglobin synthesis. Such predicates are called blank. Al- Even within the restricted class of arguments at issue in this though blank predicates are recognizably scientific in character article, a variety of phenomena can be discerned that must be (in the latter case, biological), they are unlikely to evoke beliefs accounted for by any adequate theory of category-based induc- that cause one argument to have more strength than another. tion. Each phenomenon signals the importance of a given vari- In summary, the theories discussed below bear on categorical able in argument strength when other variables are held more arguments involving natural kinds and blank predicates. An or less constant. The phenomena should thus be conceived as example of such an argument is tendencies rather than strict laws determining confirmation. Mosquitoes use the neurotransmitter Dihedron. We now present 13 such phenomena and illustrate each with a Ants use the neurotransmitter Dihedron. contrasting pair of arguments. The first argument in each pair Bees use the neurotransmitter Dihedron. (3) is claimed to be stronger than the second, in conformity with the phenomenon that the pair illustrates. At the end of this sec- Henceforth, the term argument is to be understood in the fore- tion, we describe a study that empirically documents all of these going sense. claims about relative argument strength. Terminology and Notation Phenomena Concerning General Arguments We assume that subjects (and experimenters) largely agree Let general argument PI... Pn/C be given. with each other about facts related to the hierarchical level of natural-kind concepts. To illustrate, wide agreement is presup- Phenomenon 1 (premise typicality). The more representa- posed about the following judgments: tive or typical CAT(Pt)... CAT(Pn) are of CAT(C), the more P{ ... Pn confirm C. Because robins are more typical than pen- 1. FALCON and PELICAN are at the same hierarchical level; guins of BIRD, this phenomenon is illustrated by the following 2. BIRD is one level above both FALCON and PELICAN; and 3. ANIMAL is one level above BIRD. pair of arguments: Robins have a higher potassium concentration in We cannot presuppose universal agreement about such levels. their blood than humans. For example, some subjects might take BIRD-OF-PREY to be one All birds have a higher potassium concentration in (5a) level above EAGLE, whereas others (who make fewer distinc- their blood than humans.
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