They include Magda B. Arnold (1903–2002), an American psychologist who developed the appraisal theory of emotions; Richard Lazarus (1922–2002), an American psychologist who specialized in emotion and stress, especially in relation to cognition; Herbert A. Simon (1916–2001), who included emotions into decision making and artificial intelligence; Robert Plutchik (1928–2006), an American psychologist who developed a psychoevolutionary theory of emotion; Robert Zajonc (1923–2008) a Polish–American social psychologist who specialized in social and cognitive processes such as social facilitation; Robert C. Solomon (1942–2007), an American philosopher who contributed to the theories on the philosophy of emotions with books such as What Is An Emotion?