The Surgeon General's staff administered intelligence and personality tests during World War 1 to the almost two million recruits of the American Expeditionary Force. This effort had two unexpected effects on psychology.
First, the results of the tests were discouraging. "Besides weeding out 8,648 recruits for mental insufficiency, the tests had also determined that the average mental age of these men--and by extension the nation--was thirteen years and one month. In other words, the average American was about as smart as a young teenager. What followed was an orgy of mental self-improvement [among the citizens of our country, including a tremendous dissemination of information about psychology to the general public], which lasted until the stock market crash, after which few had time for anything but material self-improvement."
The second effect of the testing was on the development of personality tests. The soldiers were given the Wordsworth Personal Data Sheet, a 125-question inventory, that was supposed to detect personalities that would crumble under fire. While the test was a failure at predicting performance under fire, it was the first attempt to quantify personality, and as such was hailed as revolutionary by psychologists. The test implied that human personality was measurable "which was a godsend to psychologists caught between Behaviorism's penchant for rats, and the untestable models of psychoanalysis. By the mid-Thirties there was an abundance of these diagnostics, ranging from Hermann Rorschach's inkblots (1921) to the Minnesota Multiphasic Personality Index and the Thematic Apperception Text..."