World War I (1914 - 1918). Psychology was first used widely in the First World War in 1917, when mass psychometric testing was carried out by the US Army. Psychologists also studied 'shell shock' or war neuroses (later recognised as Post Traumatic Stress Disorder or PTSD)
It could be argued that applied psychology was effectively born during World War I. For instance, studies of fatigue in munitions factories were the first major industrial psychology studies to be conducted. Psychologists also worked on specific programs, like for instance, the selection of hydrophone operators best suited for 'hearing submarines'. According to Hearnshaw (The Shaping of Modern Psychology, 1989), the work of psychologists during World War I “helped to establish the claims of applied psychology, and led to its continuation on a still small, but nevertheless significant, scale” (p. 200).