James W. Pennebaker’s interest in word counting began more than 20 years ago, when he did several studies suggesting that people who talked about traumatic experiences tended to be physically healthier than those who kept such experiences secret. He wondered how much could be learned by looking at every single word people used  even the tiny ones, the I’s and you’s, a’s and the’s.

That led Dr. Pennebaker, a professor of psychology at the University of Texas, down a winding path that has taken him from Beatles lyrics (John Lennon’s songs have more “negative emotion” words than Paul McCartney’s) all the way to terrorist communications. By counting the different kinds of words a person says, he is breaking new linguistic ground and leading a resurgent interest in text analysis.

Take Dr. Pennebaker’s recent study of Al Qaeda communications  videotapes, interviews, letters. At the request of the F.B.I., he tallied the number of words in various categories  pronouns, articles and adjectives, among others.

He found, for example, that Osama bin Laden’s use of first-person pronouns (I, me, my, mine) remained fairly constant over several years. By contrast, his second-in-command, Ayman al-Zawahri, used such words more and more often.