Generics and Generalizations

1. Generics and Generalizations Sarah-Jane Leslie Princeton University

2. Generics • “Tigers have stripes” is a generic sentence • It expresses a claim about tigers in general, not about a particular tiger • It does not mean that all tigers have stripes • Perhaps it is better understood as most tigers have stripes, or generally, tigers have stripes

3. Existing Accounts of Generics • Alternatively, perhaps it could be understood as equivalent to “all normal tigers have stripes” • Many variations on such an account have been offered in the literature, e.g. Pelletier and Asher; Asher and Morreau; Greenberg

4. Existing Accounts of Generics • “Birds lay eggs”, however, is of the same form as “tigers have stripes”, but it cannot be paraphrased with “most” or “generally” • It is also not true that all normal birds lay eggs; there is nothing abnormal about male birds • Attempts have been made to rescue these accounts via quantifier domain restriction

5. Existing Accounts of Generics • This is not a trivial task, since the domain must be restricted to female birds, yet “birds lay eggs and are female” is not true • And if we can restrict the domain to females, why can we not restrict it to males? • This would render true “birds don’t lay eggs”

6. The Elusive Generic • A bigger problem arises from sentences such as “mosquitoes carry the West Nile virus” • Less than one percent of mosquitoes do so • There is something rather abnormal about mosquitoes that carry it • No non-circular domain restriction is forthcoming

7. The Elusive Generic • One might be tempted at this point to think that generics of the form “Ks are F” are true if just some Ks are F • This fails to explain the falsity of generics such as “dogs have three legs” • Or “birds are female”

8. The Elusive Generic • Perhaps generics are true if the predicated property is possessed by only the kind in question • Perhaps this explains generics such as “mosquitoes carry the West Nile Virus” • There is a reading of the sentence to that effect: “It is mosquitoes that carry the West Nile Virus”

9. The Elusive Generic • On that reading, one could disagree with the assertion by claiming that deer ticks also carry the virus • There is another very natural reading, though, on which there is no incompatibility between the truth of “mosquitoes carry the West Nile Virus”, and “deer ticks also carry the West Nile Virus”

10. The Elusive Generic • Unique possession of the predicated property by a kind is also not in general a sufficient condition for the truth of a generic • “Humans are one-legged” • We are the only species that has one-legged members • “Dogs are three-legged”

11. The Complexity of Generics • As this brief discussion should indicate, the truth conditions of generics will be rather complicated • Indeed, current accounts of generics use everything from iterated modalities, to comparative probabilities, to non-standard logics

12. Cohen’s Account • Absolute generics: (Birds lay eggs) • ‘Ks are F’ is true iff the probability that an arbitrary K that satisfies some predicate in Alt(F) also satisfies ‘is F’ is greater than .5 • Relative Generics: (Mosquitoes carry the WNV) • ‘Ks are F’ is true iff the probability that an arbitrary K that satisfies some predicate in Alt(F) satisfies ‘is F’ is greater that the probability that an arbitrary member of Alt(K) that satisfies some predicate in Alt(F) satisfies ‘is F’

13. The Simplicity of Quantifiers • Compare Cohen’s account of generics to the truth conditions of “all Ks are F”: • “All Ks are F” is true iff {x: x is a K}  {x: x is F}

14. Generic Acquisition • The asymmetry in complexity between the truth conditions for generics and for explicit quantifiers such as “all” becomes all the more remarkable when we consider data from language acquisition. • Generics are easier for children to acquire!

15. Generic Acquisition • Generics appear in children’s speech long before explicit quantifiers do • Hollander, Gelman, and Star (2002) found that, under some circumstances, three-year-olds interpret explicitly quantified statements as though they were generics

16. Hollander, Gelman and Star • Hollander et al (2002) tested young children’s comprehension of “all”, “some” and generics • Participants were asked yes/no questions that were either generics, or contained “all” or “some” • Wide-scope and Narrow-scope properties

17. Adults

18. Four year olds

19. Three year olds

20. Hollander, Gelman and Star • They found no significant difference between three year olds’ answers to the three different question types • They also found no significant difference between the three ages groups’ responses to generics • Thus it appears that three year olds treat “all” and “some” like generics! • Sesame Street

21. The Unmarked Generic • This is all the more puzzling considering that there is no dedicated, articulated operator associated with generics • Children, it would seem, would not only have to learn an incredible complex set of truth conditions, they would have to associate such conditions with the absence of a quantifier

22. Default Generalizations • How, then, do children ever acquire generics? • I suggest that generics express the cognitive system’s most primitive generalizations • The capacity to generalize predates the acquisition of language • There must be some mechanism responsible for pre-verbal generalizations • I suggest generics give voice to these most primitive generalizations

23. Default Interpretations • This explains why there is no articulated generic operator • The conceptual system may be disposed to form representations in a particular way, and will do so in the absence of instructions to the contrary • These contrary instructions must be explicitly given (marked), but no instructions need be given for the default approach to proceed (unmarked)

24. Default Interpretation • Chomsky notes that in the absence of the the explicit word “down”, “climbed” is interpreted as “climbed up” • “John climbed the mountain/tree/etc” • Perhaps we have a default way of thinking about climbing, of which the unmarked case takes advantage

25. The Unmarked Generic • It is conjectured that no natural language has a dedicated generic operator; certainly there are no known languages that do • This is an odd fact, and should be explained by a theory of generics • I propose that generics are unmarked generalizations; they reflect the conceptual system’s default manner of generalizing

26. The Unmarked Generic • A generalization goes beyond particular observed instances to say something about a whole category • On this view, quantifiers like all, some, most, etc, contribute instructions that the generalization should be made in a specific non-default manner • Generics do not contribute any instructions; the conceptual system forms the given generalization according to its default approach

27. Generalizations • Generalizations are fundamental to our ways of obtaining and retaining information about the world • Graham, Kilbreath, and Welder (2001) found that 12-14 month old infants are willing to generalize non-obvious properties (such as rattling when shaken) on the basis of both perceptual similarity, and on the basis of labeling • Baldwin, Markman, and Melartin (1993) suggest that infants as young as 9 months were willing to make similar generalizations

28. Generalizations • Thus the inclination to generalize, while aided by language, does not depend on language, but is rather an early developing cognitive disposition • There must be a basic, pre-linguistic cognitive mechanism that forms these generalizations • I claim it is this mechanism that underwrites our comprehension of generics

29. Default Generalizations • Explicit quantifiers serve to direct the cognitive system to deviate from its most primitive, default means of generalizing • The unmarked generic allows the default mode to proceed • This is why no language contains an articulated generic operator

30. Characterizing the Mechanism • I claim that the strange truth-conditional behavior of generics can be traced to what are plausibly quirks and biases in this mechanism • I will highlight four main features of the mechanism

31. Characteristic Dimensions • This mechanism needs to be an efficient information gathering mechanism • It takes advantage of regularities in the world • For example, animal kinds are very similar to each other at a level of abstraction

32. Characteristic Dimensions • I suggest the mechanism operates by identifying these “Characteristic Dimensions,” and then seeking to locate a value along them for a given kind • Since reproduction is such a dimension for animal kinds, the egg-laying ducks supply a value, and the generic that attributes this value to the kind duck is thereby judged true

33. Striking Information • If the property being generalized is striking, horrific, appalling, then it takes only a few instances of the kind to possess it for the generalization to be true • Mosquitoes carry the West Nile Virus • Sharks attack bathers • Pitbulls maul children • Tigers eat people

34. Striking Information • Of course we do not generalize such properties to all kinds with members that possess them • For example, we do not accept ‘animals carry the West Nile virus’, or even ‘insects carry the West Nile virus’ • These kinds are overly inclusive; there is a cost to such overgeneralizing

35. Striking Information • Instead the mechanism looks for a good predictor of the property in question • I suggest that what makes a kind a good predictor for a striking property F is that its members share a nature that at least disposes them to be F • It matters that the virus-free mosquitoes are capable of carrying the virus, and that the sharks that don’t attack bathers are nonetheless disposed to do so

36. Other Information • If a property is not striking, and is not a value along the kind’s characteristic dimensions, then we require that the majority of the kind possess it for the generic to be true • “Barns are red”, “Cars have radios” • These would not be true if only a few members of the kind had the property

37. Positive Counterinstances • Among the members of the kind that do not possess the generalized property, it matters how they fail to possess it • In particular, it matters whether they simply lack it, or possess an equally vivid, positive property in its place

38. Birds and Eggs • I claim that we are far more prepared to hold on to generics in the face of negative counterinstances than positive ones • Suppose that I induce that birds, being animals, have reproduction as a characteristic dimension, and fill in laying eggs as the value • Male birds simply do not lay eggs; they are negative counterinstances and so I retain my generalization • I would react differently were I to learn of some birds that bore live young

39. Peacocks • “Peacocks have fabulous blue tails” is a true generic • Female peacocks, drably lacking tails, do not count against this claim • Were they to have pink tails, however, the claim would be weakened to “Peacocks have fabulous pink or blue tails”

40. An Account of Generics • We can describe the situation in which a generic “Ks are F” is true: • The counterinstances are negative, and: • If F lies along a characteristic dimension for the Ks, then some Ks are F • If F is striking, then some Ks are F and the others are disposed to be F • Otherwise, the majority of Ks are F