E.L. Thorndike, who worked from the late nineteenth century until the middle of the twentieth century, is attributed with being the father of educational psychology. However, the history of the field can be traced further than that. Greek and Roman philosophers wrote about the importance of a good education and the values that lead to a good education. The necessity of education was being written about in the sixteenth and seventeenth centuries.
One of the earliest developments in educational psychology is E.L. Thorndike's theory of connectionism. This theory states that connections are formed between stimuli and responses. Thorndike postulated that people with higher intellect could form more bonds than those of lower intellect. A French contemporary of Thorndike, Alfred Binet, developed an intelligence assessment test to determine the mental abilities of students. In the later half of the twentieth century, Swiss biologist Jean Piaget studied how children learn. American educator Benjamin Bloom created a classification system for educational goals.
Psychology is by nature a cumulative subject. That means that each new topic in the field is built on what came before it. A student can not understand one specific issue in psychology unless he studies the topic that preceded it. Because of psychology's cumulative nature, it is necessary to study different theories. Even if a theory is out of date or has been disproved, a student must still learn the theory to understand the theories that followed it.
Given psychology's cumulative nature, any psychological research must build on previous theories. To make any progress, researchers must build on existing theories, or challenge them by formulating new ideas that better explain questions. An example of a theorist whose works have been challenged is Sigmund Freud. Freud proposed that human motivation is based on either sexual desires or aggression. His students did not agree with their mentor, and several of Freud's followers formulated their own theories to explain why humans are motivated to act in the ways that they do. One student, Erik Erikson, theorized that human relationships are a strong motivating factor for individuals' behavior.
Psychology is not an exact science, such as biology or chemistry. Exact sciences rely on measurements to determine results of experiments. In contrast, psychology deals with things that can not be measured, such as happiness. Psychologists are constantly trying to overcome this issue by postulating new theories that will help them better understand human behavior. There might be two theories released at the same time regarding the same subject. Therefore, a student must study both of these theories to understand the entire issue.