 During World War I, Wallon was mobilized as an army medical officer and became interested in neurology.In 1920 he became a junior lecturer at the Sorbonne, and then in 1925 attained his Ph.D. (Docteur ès lettres) with a thesis on "the turbulent child".He was named director of studies at the École Pratique des Hautes Études in 1927 and created the Laboratory of Pediatric Psychobiology ("laboratoire de psycho-biologie de l'enfant") at CNRS, where Paul Diel came under his direction upon joining the laboratory in 1945.