Trained as a personality/social psychologist, Gail Hornstein has published widely in professional journals on such topics as conversational style in close relationships; the transition from work to retirement; the development of quantification in American psychology; and psychology's problematic relations with psychoanalysis. Her research has been supported by grants and fellowships from the National Library of Medicine; the American Council of Learned Societies; the National Science Foundation; the National Endowment for the Humanities; and the Marion and Jasper Whiting Foundation. She has been awarded visiting fellowships at the Bunting Institute, Radcliffe College; the History of Science Department, Harvard University; Clare Hall and the Centre for Research in the Arts, Social Sciences, and Humanities, Cambridge University; Magdalen College, Oxford University; and the School of Advanced Study, University of London. She chaired Mount Holyoke's Women's Studies Program for seven years, and was the founding director of the Five College Women's Studies Research Center for its first ten years.

Hornstein's current research is broadly concerned with the history of 20th century psychology, psychiatry, and psychoanalysis. Her biography To Redeem One Person Is to Redeem the World: The Life of Frieda Fromm-Reichmann (Free Press, 2000) tells the tale of a pioneering psychiatrist who dedicated her life to treating very disturbed patients. Publisher's Weekly called the book "dazzling and provocative," and it has been translated into Spanish and reviewed in more than two dozen popular and professional publications. Hornstein notes, "One goal of the book is to show that despite the widespread use of somatic treatments—such as medication, electroshock, and lobotomy—psychotherapy can be used to treat even the most severe forms of mental disturbance."

Unlike most scholars who study mental illness, Hornstein has always been as interested in patients' experiences as in doctors' theories. Her Bibliography of First-Person Narratives of Madness (now in its 4th edition) lists more than 700 titles, and her new book Agnes's Jacket: A Psychologist's Search for the Meanings of Madness (Rodale, 2009) suggests a provocative reframing of mental illness based on patients' own ideas. From Agnes Richter, who stitched an autobiographical text into every inch of the jacket she created in a 19th-century German asylum, to the hundreds of other patients who have managed to get their stories out, Hornstein shows how first-person accounts can help to bridge the gulf between the way medicine explains psychiatric illness and the experiences of those who suffer. Taking us inside the vibrant underground network of “psychiatric survivor groups” all over the world, where patients work together to unravel the mysteries of madness and help one another recover, Agnes's Jacket offers a whole new way of understanding one another and ourselves.

Hornstein teaches Theories of Personality; First-Person Narratives of Madness; Research Methods in Psychology; Qualitative Methods; Psychology of Women; and Seminar in the History of Psychology.