Lawrence Kohlberg followed the work of Jean Piaget in the field of developmental psychology. Kohlberg's theory had three levels, with two sub stages in each level. The levels are Pre-conventional, Conventional, and Post-conventional. Sigmund Freud is considered the primary originator of psycho-analysis. Abraham Maslow constructed his well-known hierarchy of needs.

Sumner recieved his PhD in psychology in 1920 from Clark University becoming the first African American to receive a PhD in psychology. Kenneth Clark was the first African American president of the APA, Mary Calkins was the first female president of the APA, and William James was the first American to receive a PhD in psychology.

Eysenck, who at the time of his death was the most cited living psychologist in the world, promoted the idea of extraversion, neuroticism, and psychoticism as the three personality traits or dimensions which, in combination, made up the majority influence in personality and psychological development.

Joseph Wolpe was a behavioral psychologist most famous for developing the therapeutic technique called systematic desensitization. The experiment and subsequent development of the theory of learned helplessness in dogs was performed by Martin Seligman.

Mary Ainsworth, whose studies of attachment models in children drastically changed the way behavioral and developmental psychologists viewed the child-parent bond, developed her Strange Situation test to demonstrate how different parenting styles and different childhood dispositions towards parents relate to later social behaviors.

The Big 5 Personality Inventory was co-developed in an effort to determine which personality traits were most essential in describing adult behaviors and attitudes.

Harlow's experiments on attachment and bonding in rhesus monkeys formed the backbone of companionship and caregiving as necessary elements in social development, and would influence the works of humanistic psychologist Abraham Maslow.

In addition to the development of REBT, Ellis is seen as one of the founders of the American "sexual revolution", having authored and co-authored several hundred scientific papers and over 75 books dedicated to the two subjects.

The language acquisition device is the innate “tool” in the brain that allows children to learn language and grammar easily. Noam Chomsky proposed that language learning cannot be totally dependent on observation and reward—since young children learning to speak make errors and generate novel expressions that they have never heard anyone else say (e.g. a parent is unlikely to have said “I wants cookie yesterday” or “do you think dinosaurs liked candy?” in front of the child). 

The correct answer is Fritz Perls. Perls and his wife were attributed with bringing Gestalt psychology to the United States. Their methods of psychoanalysis offered an alternative to the traditional psychoanalytic perspective that was already being offered.