- 1901 - Sigmund Freud published The Psychopathology of Everyday Life.
- 1903 – John B. Watson graduated from the University of Chicago; his dissertation on rat behavior has been described as a "classic of developmental psychobiology" by historian of psychology Donald Dewsbury.
- 1903 – Helen Thompson Woolley published the first dissertation on sex differences, The Mental Traits of Sex.
- 1904 – Charles Spearman published the article General Intelligence in the American Journal of Psychology, introducing the g factor theory of intelligence.
- 1905 – Alfred Binet and Theodore Simon created the Binet-Simon Scale to identify students needing extra help, marking the beginning of standardized psychological testing.
- 1905 – Edward Thorndike published the Law of Effect.
- 1906 – The Journal of Abnormal Psychology was founded by Morton Prince, for which Boris Sidis was an associate editor and significant contributor.
- 1908 - Sigmund Freud published the paper On the Sex-al Theories of Children, introducing the concept of penis envy; he also published the paper 'Civilized' Sex-al Morality and Modern Nervous Illness.
- 1908 - Wilfred Trotter published the first paper explaining the herd instinct.
- 1909 – Sigmund Freud lectured at Clark University, winning over the U.S. establishment.
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