Considering the development of children considered to be weak, Zazzo proposed the concept of "oligophrenic heterochrony" in order to show that this development, compared with that of normal children, occurred at various speeds, according to the particular psychobiological sector concerned.
The majority of research which Zazzo produced between 1950 and 1980 centered on what he regarded as "the principal problem of psychology"—that of the identity: how does a person's psyche build itself?
On his return to France, Zazzo began working for the CNRS and integrated the laboratory of Child Psychobiology and the Practical School of the Higher Studies.
When the Germans invaded Paris during World War II, he directed the laboratory of psychopathology of the Henri Rousselle Hospital.
He published his first book during the War, which was devoted to a study of the pioneers of American psychology (1942), before entering the French Resistance.