[3] He has held the following positions:[1] Over the course of his career, Pennebaker has studied the nature of physical symptoms, health consequences of secrets, expressive writing, and natural language, and has received grants from the National Science Foundation, the National Institutes of Health, the Templeton Foundation, the U.S. Army Research Institute, and other federal agencies for studies in language, emotion, and social dynamics.

"[4][6] In the mid-1990s, he and colleagues developed the Linguistic Inquiry and Word Count (LIWC; pronounced "Luke"), a computerized text analysis program that outputs the percentage of words in a given text that fall into one or more of over 80 linguistic (e.g., first-person singular pronouns, conjunctions), psychological (e.g., anger, achievement), and topical (e.g., leisure, money) categories.

It builds on previous research linking language patterns with mental states and traits but is more efficient and objective than hand coding methods.

[4][6] Pennebaker and his collaborators have used LIWC to analyze the language of Al Qaeda leaders and of political candidates, particularly in the 2008 United States presidential election.

[7][8] Pennebaker blogs with associates on what text analysis reveals about political leaders at Wordwatchers: Tracking the language of public figures,[4] and Pennebaker Conglomerates, Inc. offers free LIWC-based text analysis tools online, including a language style matching calculator and a language-based application of the Thematic Apperception Test.