Rama also wonders about other kinds of metaphors, ones that don't obviously fall into the bouba/kiki category. In his current favorite example, Shakespeare has Romeo declare Juliet to be "the sun." There is no obvious bouba/kiki-like dynamic that would link a young, female, doomed romantic lead with a bright orb in the sky, yet the metaphor is immediately sensible to anyone who hears it.
A few months ago, when we ran into each other at a conference where we were both speaking, I made a simple suggestion to Rama about how to extend the bouba/kiki idea to Juliet and the sun. Suppose you had a vocabulary of only 100 words. (This experience will be familiar if you've ever traveled in a region where you don't speak the language.) In that case, you'd have to use your small vocabulary creatively to get by. Now extend that condition to an extreme. Suppose you had a vocabulary of only four nouns: kiki, bouba, Juliet, and sun. When the choices are reduced, the importance of what might otherwise seem like trivial synesthetic or other elements of commonality is amplified.
Juliet is not spiky, so bouba or the sun, both being rounded, fit better than kiki. (If Juliet were given to angry outbursts of spiky noises, then kiki would be more of a contender, but that's not our girl in this case.) There are a variety of other minor overlaps that make Juliet more sunlike than bouba-ish.
If a tiny vocabulary has to be stretched to cover a lot of territory, then any difference at all between the qualities of words is practically a world of difference. The brain is so desirous of associations that it will then amplify any tiny potential linkage in order to get a usable one. (There's infinitely more to the metaphor as it appears in the play, of course. Juliet sets like the sun, but when she dies, she doesn't come back like it. We can all only wish that she really were more like the sun in this way. Or maybe the archetype of Juliet always returns, like the sun—a good metaphor breeds itself into a growing community of interacting ideas.)
I am not a Shakespeare scholar by any means, but it seems to me that the Bard didn't generally aim for the outer reaches of his vocabulary when he set down a metaphor. His vocabulary was large—almost 30,000 words—but he usually chose small, common words, although in shocking juxtapositions, to describe Juliet, Hamlet, or anything else important.
Likewise, much of the most expressive slang comes from people with poor or inapplicable formal education making creative use of the words they know. This is true of pidgin languages, street slangs, and so on. The most evocative words are often the most common ones that are used in the widest variety of ways. Yiddish: Nu? Spanish: Pues. . . .
One reason the metaphor of the sun fascinates me is that it bears on a conflict that has been at the heart of information science since its inception: Can meaning be described compactly and precisely, or is it something that can emerge only approximately from statistical associations between large numbers of components? Mathematical expressions are compact and precise, and most early computer scientists assumed that at least part of language ought to display those qualities too. In my August 2006 column, I described how statistical approaches to tasks like automatic language translation seem to be working better than compact, precise ones. I also argued against the probability of an initial small, well-defined vocabulary in the evolution of language and in favor of an emergent vocabulary that never became precisely defined. There is, however, at least one more possibility I didn't describe in the August column: Vocabulary could be emergent, but there could be an outside factor that initially makes it difficult for a vocabulary to grow as large as the process of emergence might otherwise prefer.
The bouba/kiki dynamic, along with other similarity-detecting processes in the brain, can be imagined as the basis of the creation of an endless series of metaphors, which could correspond to a boundless vocabulary. But the metaphor of the sun, if this explanation is right, might come about only in a situation where the vocabulary is at least somewhat limited. Imagine that you had an endless capacity for vocabulary at the same time that you were inventing language. In that case you could make up an arbitrary new word for each new thing you had to say. A compressed vocabulary might engender less arbitrary, more evocative words.
Maybe the modest brain capacity of early hominids was the source of the limitation of vocabulary size. Whatever the cause, an initially limited vocabulary might be necessary for the emergence of an expressive language. Of course the vocabulary can always grow later on, once the language has established itself. Modern English has a huge vocabulary. Try Googling "hystricine."