If I were going to write a whole book about a part of speech, I'm not sure I'd go for pronouns. Verbs, yes. Nouns, too, though that might be too obvious. Prepositions would be a dark-horse candidate, with all the subtleties they reveal about relationships.
"Hidden inside language are small, stealthy words that can reveal a great deal about your personality, thinking style, emotional state, and connections with others," he writes. "These words account for less than 0.1 percent of our vocabulary but make up more than half of the words commonly used. Your brain is not wired to notice them, but if you pay close attention, you will start to notice their subtle power."
This book is rooted in work Dr. Pennebaker did during the 1980s. He and his researchers discovered that if people who had been through traumatic experiences kept them secret, they were likely to have more health problems than people who were more open about their traumas. This led Pennebaker to experiment with having people write about the troubling chapters in their lives. He found that this exercise really did help.
Along with this insight came another set of findings. Pennebaker and his team developed a computer program to analyze his subjects' use of language and discovered that patterns of use of pronouns revealed vast amounts of information about them. "Word use was associated with almost every dimension of social psychology I studied," Pennebaker writes.
Linguists distinguish between content words and function words, such as pronouns, articles, and other words that bolt the content words together into meaningful communication. Pennebaker has found differences between the sexes: Women use more pronouns, notably "I"; men use the definite article more, presumably because they are more likely to say things like, "I think the problem is with the left front wheel." (This analysis is not without controversy.)
Pennebaker has found that subordinates tend to use "I" relatively more often than those in charge, and that people who are lying tend to use "I" less.
Pennebaker's analysis of presidential rhetoric has gotten a fair bit of attention. President Obama has drawn fire from some critics for alleged overuse of "I." This is seen as a sign of arrogance. But the Pennebaker analysis shows that Mr. Obama uses the first-person singular less than any other president since Harry Truman. So why do so many people think just the opposite?
Pennebaker's analysis ascribes this to the way we process information selectively. For those predisposed to consider the president arrogant, every little "me" he utters is more evidence. And yet, given the (admittedly counterintuitive) idea that people who see themselves as on top use first-person singular less often than people who see themselves as subordinate, it may be fair to say that Obama's speech patterns do indicate self-confidence, which his critics may inevitably interpret as arrogance.
As Ben Zimmer put it in The New York Times, "[T]he knock on Obama may indicate that listeners can properly discern his self-confidence (along with what Pennebaker calls his 'emotional distance') but then attribute this quality to precisely the wrong details of his speaking."