Brownian thought space

Cognitive science, mostly, but more a sometimes structured random walk about things.

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Wednesday, January 26, 2011

Why do some language universals have exceptions?

In a recent article, Evans & Levinson argue that there are no language universals, because for almost any universal, one can find exceptions. Of course, if you're a Chomskyan-like linguist, the obvious simple response is something like this being another case of failing to draw a competence/performance distinction. But still, one needs an explanation of why we find some rare forms; and here is (another) general possibility.
(Sidenote on Evans & Levinson: Imagine we went around observing living species. We would see a bewildering diversity, and I'm quite confident that for any "rule" relating to the phenotypes, there would always be some exceptions. E.g., swimming mammals, flying snakes, flightless birds, carnivorous plants etc. But from this, should we conclude that there is no underlying "universal" organizing principles? No - we know that there is an underlying organizing principle, but that the variation can sometimes make it hard to see. But it's a better explanation of the facts than to assume that each species somehow arises independently of the other, in response to the local environmental niche. [Of course, this analogy might not be perfect; living systems do evolve; while it's not clear if languages do/have done in quite the same way.])
The possible explanation for rare forms arises from work on language typology and language change. In the former, one finds broad generalizations, sometimes with exceptions. For example, Jenny C. recently told us all about Greenberg's Universal 18, by which Adjective-Noun and Noun-Numeral combinations are forbidden (so you can't say something like "red balls three"). It turns out that, in a large survey of languages, about 4% actually do show this pattern (on the surface).
Now we also know that languages change; and in some cases languages change along gradients. Note that this sense of 'gradient' is different from some others, which equate 'gradient' effectively with non-discrete (and hence non-symbolic?) views of language. In the sense used in this post, a gradient is a hierarchy of constituents to which a certain rule can be applied at different levels of the hierarchy. To give an example (thanks to Jenny.C.), consider the hierarchy of DP definitiveness:
Pronoun > Definite DP > Indefinite DP/Quantifier > Wh-phrase
where X > Y implies that Y is higher up in the hierarchy than X. So, for example, if a certain grammatical rule (like agreement) applies to Y, it must also apply to X, but not vice-versa. (In the above example, one can think of the left-to-right progression as getting successively more unspecific about the intended referent.)
Why should a language change from left to right? A general answer (e.g. in this paper) is that it is due to a tendency for generalizations. I.e., if in generation 1 agreement is only on pronouns, and there is even a little evidence for generalizations for definite DPs, over time the regularization bias will quickly move the generalization to the right in the hierarchy. Presumably, there might not even need to be any evidence - if the input is sparse and the learner is trigger-happy, s/he might hypothesize that a generalization holds at a level higher than the one observed in the data.
But there are two possible scenarios for such gradients like X > Y > Z. One is that this sequence is self-closing, such that X > Y > Z > X ..., much like Rock-Paper-Scissors. In such a scenario, one would expect that, starting from different positions in the hierarchy, and changing at different rates, at any given time one might expect to find all three kinds of systems.
However, if X > Y > Z, and that's the end of the story, then, over time, the elements higher up in the hierarchy will tend to dominate, and Xs will tend to be infrequent or disappear.
(A third possibility is that, for different reasons, learners might be able to move to both the right and the left of a hierarchy. In this case too, languages would be expected to be of all three kinds.)
In essence, the reason why certain grammatical features might be rare is that they represent what a complex system theorist might call repellers - regions that a dynamically evolving system avoids. That is, gradients of change might mostly lead away from some linguistic forms, making them universally rare.

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