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The flying purple-people-eater, and other compound words



What’s wrong with the following sentence: I saw a one-eyed, one-horned, flying purple-babies-eater? It sounds off doesn’t it? If you have heard the song I am referencing I’m sure you would quickly correct me—it’s a purple-people-eater not a purple-babies-eater. But even if you hadn’t heard the song you would probably say, don’t you mean a flying purple-baby-eater?

The error our ears hear in the sentence is built into the way we use language. Steven Pinker explains this word quirk in his book The Language Instinct with a theory of word structure and mental storage. He notes that the noun in a compound word, such as people in purple-people-eater, must be a stem. A stem is the simplest unit of a word that can be manipulated into different parts of speech when certain rules are applied to it. Book would be the stem of books, table the stem of tables, and owl the stem of owls. What do these stems have in common? They are all regular nouns in the sense that forming each of their plurals follows a simple rule—add an s. 

We all know about irregular nouns then. The plural forms of irregular nouns can’t be predicted by a rule like add an s. Here are just a few examples: calf-calves, foot-feet, and child-children. Just like when you were learning irregular verb conjugations in Spanish class, these irregular English plural forms must simply be memorized. Since by definition there is no rule for how to form irregular plurals, both foot and feet must be stored in our memories separately as two different concepts! The same is true for calf and calves, and goose and geese.
 
Steven Pinker would say that all four of these words, calf, calves, goose, geese, are stored in our mental rolodex as stems. Since compound words are only formed from stem words, plural forms of irregular nouns are available for compounding while plural forms of regular verbs are not. This explains why purple-people-eater sounds okay, but purple-babies-eater definitely does not. 

Whether or not word stems actually exist and are stored as little chunks of communicative units, this rule of compounding is quite fascinating. It leaves me asking questions like “Why do we have irregular plural nouns at all?” and “Why do we intuitively know that babies-eater is wrong while baby-eater is right?” Although Pinker’s word structure/storage theory doesn’t answer these questions, it is a nifty way to conceptualize one of the many quirks of the English language.

Check out the suggested further reading for an intriguing psychological study of compound word forming in children and to find more examples of irregular plural compounds. Or better yet, think of some for yourself and post them in the comments below!

Sources and further reading
Pinker, S. (1994) The Language Instinct. Camberwell, VIC, Australia: Penguin Group.
 
Berent, I. & Pinker, S. (2007). The dislike of regular plurals in compounds: Phonological familiarity or morphological constraint? The Mental Lexicon 2(2), pp 129-181: John Benjamins Publishing Company.

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