The average English speaker knows a whopping one hundred thousand words. According to new research by psychologist James W. Pennebaker, however, the most revealing words in this impressive vocabulary are the ones we barely notice at all.

Pennebaker’s research, which he published in his new book, The Secret Life of Pronouns: What Our Words Say About Us, shows that our use of certain short, stealthy words — such as pronouns, prepositions, and articles — acts as a sort of psychological fingerprint, providing fascinating insights into our personalities and thought patterns. Over the course of his career, he has used computers to analyze word usage in everything from Twitter feeds to great works of literature, uncovering the subtle ways in which language reveals who we are.

Pennebaker first became interested in words in the 1980s, when he conducted research showing that, for victims of trauma, writing about emotional experiences for 15 minutes a day over the course of three or four days produced measurable improvements in physical health. He wondered if certain patterns of language use in these “trauma essays” could predict health improvements. Pennebaker and his colleagues created a computer program called the Linguistic Inquiry and Word Count (LIWC) that counted the number of words in any given document and placed them into categories, such as anger words, positive words, pronouns, articles, and cause-effect words. He found that those who showed the greatest improvements in physical health were the ones whose use of “self-reflection” words like “think,” “realize,” and “believe,” increased from day to day. Following this experiment, Pennebaker realized that LIWC could be applied to other types of writing too. Since then, his use of computer-based linguistic analysis has yielded intriguing insights into gender, personality, politics, and more.

Gender-based language variations were among the first topics that Pennebaker studied with LIWC. Not surprisingly, he found that there are striking differences between the language usage of men and women. After analyzing thousands of blogs, essays, and other writing samples, he found that women use first-person singular pronouns like “I,” “me,” and “my” more frequently than men; according to his research, the average woman will use about 85,000 more pronouns per year than will the average man. Pennebaker attributes these findings to the tendency of women to be more self-aware than men, as has been documented by numerous psychological studies. He also found that women use more verbs and hedge phrases (such as “I think” and “I believe”), whereas men tend to use more numbers, nouns, and words per sentence.

Pronoun use also reveals much about a person’s emotional state. Noting that depression rates are unusually high among writers, Pennebaker analyzed the published writing of 18 poets, half of whom had committed suicide. He found that suicidal poets such as Sylvia Plath used pronouns like “I,” “me,” and “my,” at much higher rates than their non-suicidal counterparts. Remarkably, however, the suicidal and non-suicidal poets used approximately the same number of negative emotion words. Pennebaker’s research suggests that pronouns are far better than descriptive words at offering windows into people’s emotional lives.

Pronoun use may even provide insights into personality. After collecting and analyzing stream-of-consciousness essays from about 8,000 college students, Pennebaker found that certain patterns in language use could predict personality traits. “Formal” writers — writers who use few “I” words and many articles, nouns, and numbers — tend to be dishonest and concerned with status and power. “Analytic” writers — those who frequently use causal words like “because” and “reason,” along with negations like “no,” “not,” and “never” — tend to get good grades and be open to new experiences. Finally, “narrative” writers — writers characterized by frequent use of pronouns and verbs — are usually outgoing and have exceptional social skills.

Based on these findings, Pennebaker has used his research for more serious applications, such as analyzing transcripts of Osama bin Laden’s interviews, letters, and articles. He found that bin Laden used “we” frequently, which is indicative of a high level of self-confidence, while his heavy use of past-tense verbs and social references suggest that he was a natural storyteller (and would have fallen into the “narrative” category above). Overall, Pennebaker concluded that bin Laden’s language demonstrated a low need for affiliation, a moderate need for achievement, and a great need for power. Pennebaker has also applied his research to analyze other public figures, using patterns in their word choice to glean insights into their personalities.

This fascinating research shows that our everyday speech is full of data, buried in the deluge of texts, emails, online posts, and conversations that define our daily lives. Those short, all-too-forgettable words that our brains readily skim over may in fact hold the keys to understanding our personalities, our emotions, our relationships, and all that makes us who we are.