One can see that in how he summarizes hundreds of research reports in cognitive science and evolutionary psychology. He describes this as a "perilous enterprise," because many of these studies are in dispute, and it's often impossible to briefly summarize complex research in ways that convey the complexity and nuance in the work. He admits that he won't satisfy all of his readers, but he notes that he has "tried to direct readers to sources where they can read about the original work and draw their own conclusions about its implications" (377-78). And, indeed, his notes with research citations take up 26 pages of small print at the back of the book.
Consequently, any careful reader of Brooks's book will have to read some of the research he cites in order to judge whether his claims are warranted or not. I foresee that when I use this book as a text in an undergraduate class, I will ask my students to do that--to read some of the original research for themselves and then assess its plausibility for supporting Brooks's arguments.
It's clear, however, that some of the early reviewers of Brooks's book have not done this. For example, in his review in the New York Times Book Review, Thomas Nagel complains: "Brooks seems willing to take seriously any claim by a cognitive scientist, however idiotic: for example, that since people need only 4,000 words for 98 percent of conversations, the reason they have vocabularies of 60,000 words is to impress and sort out potential mates." It's easy to dismiss such as claim as "idiotic" without actually scrutinizing the research that supports it.
The passage in the book that Nagel finds so "idiotic" comes in the context of Brooks's story of how Harold's parents--Rob and Julia--first met for a blind lunch date. After describing the flow of their conversation and how much they enjoyed talking with one another, Brooks cites some research in evolutionary psychology that might explain the importance of language for sexual mating:
Words are the fuel of courtship. Other species win their mates through a series of escalating dances, but humans use conversation. Geoffrey Miller notes that most adults have a vocabulary of about sixty thousand words. To build that vocabulary, children must learn ten to twenty words a day between the ages of eighteen months and eighteen years. And yet the most frequent one hundred words account for 60 percent of all conversations. Why do humans bother knowing those extra fifty-six thousand words?
Miller believes that humans learn the words so they can more effectively impress and sort out potential mates. He calculates that if a couple speaks for two hours a day, and utters on average three words a second, and has sex for three months before conceiving a child (which would have been the norm on the prehistoric savanna), then a couple will have exchanged about a million words before conceiving a child. That's a lot of words, and plenty of opportunities for people to offend, bore, or annoy each other. It's ample opportunity to fight, make up, explore, and reform. If a couple is still together after all that chatter, there's a decent chance they'll stay together long enough to raise a child.
Harold's parents were just in the first few thousand words of what, over the course of their lifetimes, would be millions and millions, and things were going fabulously. . . . (13)
The reader who checks the endnotes for this passage will see that Brooks is citing Geoffrey Miller's The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (Random House, 2000), pages 369-75. This is a book surveying recent evidence and theorizing in biology and psychology supporting Charles Darwin's theory of sexual selection--the idea that both human sexes have evolved ways of displaying cues of fitness that attract mates. The passage cited by Brooks is part of a chapter on "verbal courtship theory," which tries to explain how human verbal behavior might have evolved to make human beings more attractive to potential mates.
Miller's reasoning in this particular passage moves through eight steps. Throughout the passage, Miller uses words like "may" and "perhaps" to suggest how tentative his conclusions are. And yet Miller's arguments and evidence are impressive enough to make a powerful case for his theory that the size of a human adult's vocabulary has evolved through a history of mate choice.
Miller's first step is to point out that the typical adult vocabulary of 60,000 words is excessive, because it surpasses what we need for pragmatic communication. The most frequent 100 words account for about 60 percent of all conversation, and the most frequent 4,000 words account for about 98 percent. Why do we need all these words? If we have the word "blue," why do we need "azure," "cobalt," "sapphire," "ultramarine," "cerulean," and "indigo"?
His second step is to show that artificial languages and "pidgin" languages can satisfy the practical needs of life with very small vocabularies. In the 1920s, C. K. Ogden and I. A. Richards developed what they called Basic English. With an English vocabulary of 850 words, this language could serve all the purposes of everyday practical life. Similarly, pidgin languages are developed when people with different languages have to communicate with one another for practical purposes, and these languages have very small vocabularies. This shows that the much larger vocabularies of normal human languages are not necessary for pragmatic communication.
Miller's third step is to note an analogy to bird song. Most bird song evolves through mate choice. Some birds--such as nightingales--have a large repertoire of courtship songs (over a thousand), and this seems to be because those males who sing more songs are more attractive to mates, which seems to arise because a larger song repertoire of songs is an indicator of intelligence and learning abilities that enhance fitness.
Miller's fourth step is to note that vocabulary size varies greatly among human beings, and it seems to indicate factors of intelligence and learning ability that are highly heritable and indicators of fitness. Consequently, our ancestors would have benefited by selecting mates with large vocabularies that would have indirectly indicated fitness.
Miller's fifth step is to indicate that although most human beings are probably not consciously aware that they prefer mating with someone who has a large vocabulary, there is some evidence that couples in long-term relationships are similar to one another in the size of their vocabularies. Since intelligence testing indicates that the size of one's vocabulary correlates with one's IQ, choosing mates based on their vocabularies is an unconscious inference about their intelligence.
Miller's sixth step is to point to how, in the film Mary Poppins, the song "Supercalifragilisticexpialidocious" highlights the power of unusual words to advertise intelligence.
Miller's seventh step is to admit that his verbal courtship theory has not yet been fully tested:
To test this verbal courtship theory of vocabulary properly, we would have to find out much more about human verbal behavior than language researchers know at present. We don't know the size of typical ancestral or tribal vocabularies. We don't know whether people use more impressively obscure words during courtship. We don't know whether large vocabularies are valued directly in human mate choice. We don't know how vocabulary sizes correlate with brain size, physical health, physical attractiveness, fertility, or general fitness. Sex differences in the distribution of vocabulary sizes are rarely reported in the scientific literature (though they are perfectly well known to the Educational Testing Service that administers the SAT). (374-75)
After this concession as to the need for more study to test his theory, Miller concludes by affirming that the evidence and reasoning that he has presented should at least suggest the plausibility of explaining the large size of human vocabularies as evolving through sexual selection just like the song repertoires in some bird species.
For Thomas Nagel to flippantly dismiss all this as "idiotic" is . . . well, idiotic.