Stern coined the term intelligence quotient (IQ) and invented the tone variator as a new way to study human perception of sound.

Together with his wife, Clara Joseephy Stern, the couple kept meticulous diaries detailing the lives of their 3 children for 18 years.

When Joseph died in 1890, he left his family very little money, and William, who was studying at university, had to take up tutoring to support his sickly mother until her death in 1896.

He spent one year in Holland before moving to the United States to accept a job as a professor at Duke University, despite knowing little English.

[3] Stern studied philosophy and psychology at the University of Berlin under the guidance of Hermann Ebbinghaus where he later received his PhD in 1893.

[4] Stern and his wife, Clara, greatly influenced the area of child development and they used their three children, Hilde, Gunther, and Eva, as subjects.

He applied convergence theory to the development of language in children, stating that a child does not just mimic what is said to them but instead chooses what words that they comprehend.

[10] A student of memory research pioneer, Hermann Ebbinghaus, Stern had participants look at photographs and later asked them to recall details.

He collaborated with criminologist, Franz v. Liszt, and in 1901 conducted a study in which law students witnessed a staged classroom argument in which one protagonist drew a revolver at which point the professor stopped the mock fight.

Stern and Franz found that, in such demonstrations, the subsequent recall was poor when tension was high, leading them to conclude that emotional states could affect eyewitness testimony.

Stern noted that memory was fallible and sought ways to differentiate between intentional and unintentional falsification of testimony.