Learning the passive

If Microsoft Word had its way, passive verbs would be excised from the language. That would solve children some problems, because passive verbs are more difficult to learn than one might think, because not all verbs passivize. Consider:

*The bicycle was resembled by John.
*Three bicycles are had by John.
*Many people are escaped by the argument.

The bicycle was resembled by John: A how-to guide.

So children must learn which verbs have passives and which don't. I recently sat down to read Pinker, Lebeaux and Frost (1987), a landmark study of how children learn to passivize verbs. This is not a work undertaken lightly. At 73 pages, Pinker et al. (1987) is not Steve Pinker's longest paper -- that honor goes to his 120-page take-down of Connectionist theories of language, Pinker and Prince (1988) -- but it is long, even for psycholinguistics. It's worth the read, both for the data and because it lays out the core of what become Learnability and Cognition, one of the books that has had the most influence on my own work and thinking.

The Data


The authors were primarily interested in testing the following claim: that children are conservative learners and only passivize verbs that they have previously heard in the passive. This would prevent them from over-generating passives that don't exist in the adult language.

First, the authors looked at a database of transcriptions of child speech. A large percentage of the passive verbs they found were passives the children couldn't possibly have heard before because they aren't legal passives in the adult language:

It's broked? (i.e., is it broken?)
When I get hurts, I put dose one of does bandage on.
He all tieded up, Mommy.

Of course, when we say that the child couldn't have heard such passives before, you can't really be sure what the child heard. It just seems unlikely. To more carefully control what the child had heard, the authors taught children of various ages (the youngest group was 4 years old) made-up verbs. For instance, they might demonstrate a stuffed frog jumping on top of a stuffed elephant and say, "Look, the frog gorped the elephant." Then they would show the elephant jumping on top of a mouse and ask the child, "What happened to the mouse?"

If you think "gorp" has a passive form, the natural thing to do would be to say "The mouse was gorped by the elephant." But a child who only uses passive verbs she has heard before would refuse to utter such a sentence. However, across a range of different made-up verbs and across four different experiments, the authors found that children were willing -- at least some of the time -- to produce these new passive verbs. (In addition to production tests, there were also comprehension tests where the children had to interpret a passivization of an already-learned verb.)

Some Considerations

These data conclusively proved that children are not completely conservative, at least not by 4 years of age (there has been a lot of debate more recently about younger children). With what we know now, we know that the conservative child theory had to be wrong -- again, at least for 4 yos -- but it's worth remembering that at the time, this was a serious hypothesis.

There is a lot of other data in the paper. Children are more likely to produce new passive forms as they get older (higher rates for 5 year-olds than 4 year-olds). They taught children verbs where the agent is the object and the patient is the subject (that is, where The frog gorped the elephant means "the elephant jumped on top of the frog"). Children had more difficulty passivizing those verbs. However, a lot of these additional analyses are difficult to interpret because of the small sample sizes (16 children and only a handful of verbs per experiment or sub-experiment).

Theory 

Fair warning: the rest of this post is pretty technical.

What excites me about this paper is the theoretical work. For instance, the authors propose a theory of linking rules that have strong innate constraints and yet still some language-by-language variation.

The linkages between individual thematic roles in thematic cores and individual grammatical functions in predicate-argument structures is in turn mediated by a set of unmarked universal linking rules: agents are mapped onto subjects; patients are mapped onto objects; locations and paths are mapped onto oblique objects. Themes are mapped onto any unique grammatical function but can be expressed as oblique, object or subject; specifically, as the 'highest' function on that list that has not already been claimed by some other argument of the verb.

With respect to passivization, what is important is that only verbs which have agents as subjects are going to be easily passivized. The trick is that what counts as an 'agent' can vary from language to language.

It is common for languages to restrict passivized subjects to patients affect by an action ... The English verbal passive, of course, is far more permissive; most classes of transitive verbs, even those that do not involve physical actions, have the privilege of passivizability assigned to them. We suggest this latitude is possible because what counts as the patient of an action is not self-evident ... Languages have the option of defining classes in which thematic labels are assigned to arguments whose roles abstractly resemble those of physical thematic relations...

This last passage sets up the core of the theory to be developed in Learnability and Cognition. Children are born knowing that certain canonical verbs -- ones that very clearly have agents and patients, like break -- must passivize, and that a much larger group of verbs in theory might passivize, because they could be conceived of as metaphorically having agents and patients. What they have to learn is which verbs from that broader set actually do passivize. Importantly, verbs come in classes of verbs with similar meanings. If any verb from that set passivizes, they all will.

This last prediction is the one I am particularly interested in. A later paper (Gropen, Pinker, Hollander, Goldberg & Wilson, 1989) explored this hypothesis with regards to the dative alternation, but I don't know of much other work. In general, Learnability and Cognition go less attention than it should have, perhaps because by the time it was published, the Great Past Tense Debate had already begun. I've often thought of continuing this work, but teaching novel verbs to children in the course of an experiment is damn hard. Ben Ambridge has recently run a number of great studies on the acquisition of verb alternations (like the passive), so perhaps he will eventually tackle this hypothesis directly.

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Pinker S, Lebeaux DS, and Frost LA (1987). Productivity and constraints in the acquisition of the passive. Cognition, 26 (3), 195-267 PMID: 3677572