Q: Which of these sentences is correct? (1) “Books are what make you smarter.” (2) “Books are what makes you smarter.” Option 1 hurts my ears, while option 2 seems wrong to my friends.
A: We would choose plural verbs all the way—”Books are what make you smarter”—because the principal subject is “books.”
In a sentence starting with a singular principal subject we’d choose singular verbs: “Education is what makes you smarter.”
As we wrote in 2012, the word “what” can be construed as either singular or plural. It takes its number (singular vs. plural) from the context, and here the context is “books” (plural). Thus, “Books are what make you smarter.”
In a sentence like that, the main clause is “Books are,” and the subordinate clause, introduced by “what,” is the object of the main clause.
George O. Curme, in A Grammar of the English Language (Vol. II, 1931), uses the examples “Truth is what hurts” (singular) and “The factories are what blacken up the city so” (plural).
As Curme explains, sentences like these—written with “what” clauses as predicates—are more emphatic than if they had been written simply as “Truth hurts” or “Factories blacken up the city so.”
“The principal verb [hurts, blacken up] is stressed by putting it in an unusual position,” Curme writes, “especially by forming a predicate clause in which what is subject and the emphatic verb is predicate.”
Now, how about a sentence that starts with “what”?
In a simple sentence, with only one clause, the choice of verb with “what” is easy. Just match it with the complement: “What is your suggestion?” (singular), or “What are your suggestions?” (plural).
But when there are two clauses, as we wrote in that 2012 post, there’s some wiggle room in the choice of verbs. As we said, what’s known as “notional agreement”—the writer’s meaning—plays a role here.
You could justify either “What make you smarter are books” or “What makes you smarter is books.” In the first example, the writer regards books as “the things that make you smart,” while in the second, books represent “the thing that makes you smart.”
It’s our feeling that two singular verbs are more natural than two plural verbs when the complement—even though formally plural like “books”—represents a singular concept. So we’d choose “What makes you smarter is books.”
There’s an excellent usage note about all this in The American Heritage Dictionary of the English Language (5th ed.) We’ll underline the verbs to make the examples clearer:
“Occasionally the choice of a singular or plural verb may be used to convey a difference in meaning. In the sentence What excite him most are money and power, the implication is that money and power are separable goals; in What excites him most is money and power, the implication is that money and power are inextricably bound together.”
The dictionary continues: “When the verb in the what-clause is singular and the complement in the main clause is plural, one finds both singular and plural verbs being used. Sentences similar to both of the following are found in respected writers: What drives me crazy is her frequent tantrums; What bothers him are the discrepancies in their accounts.”