Robert Sessions Woodworth (October 17, 1869 – July 4, 1962) was an American academic psychologist of the first half of the twentieth century. A graduate of Harvard and Columbia, he studied under William James along with such prominent psychologists as Leta Stetter Hollingworth, James Rowland Angell, and Edward Thorndike. His textbook Psychology: A study of mental life, which appeared first in 1921, went through many editions and was the first introduction to psychology for generations of undergraduate students. His 1938 textbook of Experimental Psychology was scarcely less influential, especially in the 1954 second edition, written with Harold H. Schlosberg. He is known for introducing the Stimulus-Organism-Response (S-O-R) formula of behavior. A Review of General Psychology survey, published in 2002, ranked Woodworth as the 88th most cited psychologist of the 20th century, tied with John Garcia, James J. Gibson, David Rumelhart, Louis Leon Thurstone, and Margaret Floy Washburn.