The problem with introspection was that it was impossible for the researcher
to verify people’s reports about their subjective experiences.
This made it difficult to construct a formal scientific process of discovery.
So psychology turned away from introspection to a more objective approach
that emphasized controlled experiments on observable behavior.
In the first decade of the 1900s, Ivan Pavlov published his famous studies of salivating dogs
and established the principles of a type of learning that we now call classical conditioning.
At about the same time, Edward Thorndike discovered a basic principle of learning
called the “law of effect” by observing how cats learn to escape from a box.
Margaret Floy Washburn, who in 1894 became the first woman to receive a Ph.D. in psychology
published an important book on her studies of animal behavior in 1908.
This was a remarkable achievement,
considering that women did not gain the right to vote in the United States until 1920.
John Watson formalized the new, objective approach to psychology
and redefined psychology as the “science of observable behavior” in a 1913 article
launching the form of psychology called behaviorism.
John Watson and Rosalie Rayner demonstrated that the behavioral findings from animal research
could also be applied to humans through a controversial study in which they conditioned an infant to fear a white rat.
In the 1930s, B. F. Skinner built on Thorndike’s law of effect to show that behavior
could be controlled by the application of reinforcers and punishers
a process that we now call operant conditioning.
Behaviorism became the dominant approach to psychology from the 1920s to the 1960s,
though other approaches also emerged.
One of note is the Gestalt psychology movement
founded in Germany by Max Wertheimer with the study of a visual illusion in 1910.
Gestalt psychology started with the study of how we organize our perception
and from this an entire theory of the mind emerged.
When the Nazis came to power in 1933
Wertheimer and his chief followers left Germany for the United States, where they spread their ideas.