The author of this volume from 1912, William Stern (1871-1938), was a significant German psychologist, the founder of Differential Psychology, and the inventor of the first intelligence quotient. Alfred Binet, who developed the first usable intelligence test with the Binet-Simon test in 1905, indicated mental performance as an intelligence age. The test consisted of tasks with increasing difficulty that were meant to be distinctly solvable for the respective age groups. If a child could not complete all the tasks for their age group, they had a lower intelligence age; if they could also complete tasks from the higher age group, they had a higher intelligence age. William Stern related this intelligence age to chronological age and thus invented the intelligence quotient in 1912. The IQ calculation, originally developed only for children, specifically for school readiness tests, became widely used.