Rosch, Mervis, Gray, Johnson, and Boyes-Braem. Chances are, if you've taken an advanced course in cognitive psychology, you've heard those names, in that order, before, and if you study concepts, they may have been melded into one word for you. Roschmervisgrayjohnsonboyesbraem. I've said it so many times, and so rapidly, that I sometimes forget the Gray is even in there. The paper (Roschmervis[gray]johnsonandboyes-bream, 1976) deserves the fame, too. It's a Behemoth, with 12 experiments, in an age when it was still possible to publish papers with only one. The experiments range from developmental studies looking at early category acquisition, to a linguistic analysis of American Sign Language, and an experiment that determines the most general level at which category members elicit common motor programs. And all of them are designed to show one thing: for hierarchically organized (taxonomic) categories, there is a priveleged level, the basic level. Since Roschmervis and so on, it's been necessary for any theory of categorization and concepts to deal with basic level phenomena, and that hasn't always been easy.
What is the basic level? Obviously, it's the level between the superordinate and subordinate levels. Duh. Seriously though, it's a level in a conceptual taxonomy somewhere between the most general (e.g., animal, or anything more general than that) and the most specific (e.g., yellow-bellied sapsucker). Where that level is, exactly, depends on the concept. For example, the basic level for some animal concepts is somewhere around the class or order level. Thus, BIRD is a basic level concept. For others, the basic level falls somewhere further down. The basic level for most mammals, for instance, tends to fall somewhere around the family or genus level. There's no good explanation for this difference between birds and mammals, other than that American undergrads (who tend to make up the bulk of the subject pools used for concept experiments) don't have as much experience with mammals. For non-biological concepts, the basic level tends to follow distinctions of function. So, CHAIR is a basic level concept, and so are TABLE, HAMMER, and COMPUTER. At least, that's where we'll find the basic level if we're studying typical American undergraduates under typical conditions. Things start to get much more complicated when we deviate from that.
But before we get into that mess, there are at least two questions that need answering: What's so basic about the basic level, and why? To answer the first question, the basic level tends to be the first concepts young children learn1, the first names they use, and even appear in infancy (in pre-linguistic infants, in fact)2. That's not surprising since it's the level at which parents tend to label concepts when speaking to young children (a canine walking in the park is a "dog," not a "Turkish Wolfhound," when you're talking to an 18 month old). In fact, it's the level at which adults tend to label things in general conversation. It's the most general level at which it's easy to obtain conceptual priming, whether the priming is is designed to facilitate visual detection or visual categorization; the most general level at which a common motor program can be used to interact with objects; the level at which people tend to first identify an object when presented with it visually; the last concepts to drop out when we reduce the number of lexical items in a language (e.g., in the movement from spoken English to American Sign Language); the level at which information tends to be remembered over time, regardless of how general or specific the information was at the time of encoding3; and much, much more. That's the easy question to answer. The why question is not so easy.
Since the Roschandabunchofotherpeople paper's publication in 1976, there have been all sorts of attempts to explain existence and effects of the basic level. To try to describe all of them, along with the arguments or an against them, in a blog post would be... well, if you didn't fall asleep, I would. Rosch et al. explained the basic level effects by referencing cue validity. Basically (pardon the pun), they (and in one way or another, most since) believed that the basic level was the level with the largest combined within-category similarity and between-category distinctiveness. Thus, while the members of the category yellow-bellied sapsucker are more similar to each other than all birds, they are less distinctive from other birds than birds in general are from non-birds, so bird is the basic level. A related early explanation4 concerned the parts of objects. Tversky and Hemingway showed that when asked to list features of concepts, participants listed parts more often for basic level concepts than for either superordinate or subordinate level concepts. They argued that for natural kind concepts, parts are associated with their causal origins, and for artifact concepts, with their functions. Since common origins and functions tend to converge at the basic level, the basic level tends to be defined by the sharing of common parts, with basic level category members sharing a lot of properties, while members of different basic level categories do not.
There is no explanation of basic level effects that hasn't been challanged by the results of some experiment or another. Tversky and Hemingway's part-convergence explanation has been called into question by experiments showing that, at least for artificial categories, common parts are not necessary for the existence of basic level effects5. My personal favorite, due largely to my own theoretical biases, involves differences, rather than similarities. Markman and Wiskiewski6 showed that what distinguishes categories at the basic level is a higher number of "psychologically relevant" differences. In their experiments, participants listed more "alignable differences" (differences that are part of a common relational structure - e.g., wings vs. arms) for objects from different basic level concepts from the same superordinate category, than they did for subordinate concepts from the same basic level category, or different supordinates. In other words, Markman and Wisniewski showed that the basic level is important because basic level categories differ from other categories at the same level in important ways.
I like this explanation because it fits best with a "theory theory" of concepts, and I think that's important because of one very interesting basic level phenomenon: the shifting of the basic level in experts. In a set of experiments, the basic findings of which have been replicated several times (including in interesting cross-cultural research), Tanaka and Taylor7 showed that experts in a particular domain (e.g., experienced birdwatchers) tend to treat subordinate level categories like non-experts treat basic level categories, in that they are as different from each other as basic-level categories, their names are used as often as basic-level names, and their instances are recognized as rapidly as instances of basic-level categories. Thus, it appears that for experts, the basic level shifts, in their domain of expertise, to what would ordinarily be the subordinate level. Why is this? Clearly, the added domain knowledge has an effect on where the basic level lies. Part-convergence and other traditional cue-validity explanations have a difficult time explaining this effect, but the "alignable difference" explanation described above can handle it easily. As knowledge of the relevant differences between different subordinate-level categories are learned (through, e.g., the acquisition of theoretical knowledge about a domain), people are able to differentiate subordinate-level concepts better, and thus to treat it like the basic level.
So, that's the basic level. While I've obviously endorsed the "alignable differences" account of the basic level, there are other explanations out there that can explain the shift of the basic level in experts, along with the other basic level phenomena. There's really no theoretical concensus about why the basic level exists, and how it does what it does. As is often the case in concept research, it's difficult to provide rigorous tests of theories, because while natural categories in which taxonomic relations tend to exist fairly straightforwardly do not provide for a great deal of experimental control, and thus do not allow strong tests of particular theories, it is damn near impossible to produce a set of artificial categories with a structure sufficiently rich to produce taxonomic relations analogous to those in natural categories. So, we're left to try to rule out explanations rather broadly (e.g., strict similarity views vs. views that focus on differences). But because the basic level is so pervasive and important in conceptual development, learning, and use, any general theory of concepts will have to explain it, and so we have to try to study it as best we can.
1 The phenomena described in this paragraph were first demonstrated in the experiments by Roschmervisandtherestofthem, unless otherwise indicated.
2 Pauen, S. (2002). The global-to-basic level shift in infants' categorical thinking: First evidence from a longitudinal study. International Journal of Behavioral Development, 26(6), 492-499.
3 Pansky, A., & Koriat, A. (In Press). Hierarchical memory distortions: The basic-level convergence effect. Psychological Science.
4 Tversky, A., & Hemingway, K. (1984). Objects, parts, and categories. Journal of Experimental Psychology: General, 113(2), 169-197.
5 Murphy, G.L. (1991). Parts in object concepts: Experiments with artificial categories. Memory & Cognition, 19(5), 423-438.
6 Markman, A.B., & Wisniewski, E.J. (1997). Similar and different: The differentation of the basic level. Journal of Experimental Psychology: Learning, Memory, and Cognition, 23(1), 54-70.
7 Tanaka, J.W., & Taylor, M. (1991). Object categories and expertise: Is the basic-level in the eye of the beholder? Cognitive Psychology, 23, 457-482.