Infantile Autism: The Syndrome and Its Implications for a Neural Theory of Behavior (hereafter Infantile Autism) is a book written by Bernard Rimland, published in 1964. The book proposed a theory to explain the causes of autism. The book also synthesized research into autism and used Rimland's neural theory, described in the book, as a theory to explain some aspects of behavior, intelligence, and abnormality. Moreover, Infantile Autism contributed to a debate between Rimland and child psychologist Bruno Bettelheim in the 1960s over whether autism was caused by upbringing (see refrigerator mother theory) or by impaired brain development. Rimland's book convinced many autism researchers to look for abnormal psychological development.

Franz Josef Kallmann studied the biological and genetic factors of psychological disorders in Germany and the United States in the twentieth century. His studies at the New York State Psychiatric Institute in New York City, New York, focused on the genetic factors that cause psychiatric disorders. Kallmann was one of the first to use twins to study how a mental disorder is passed on by comparing the occurrence of epilepsy and schizophrenia in both fraternal and identical twins. Kallmann helped to develop and popularize the methodology of twin studies to examine the genetic component of psychiatric disorders and he showed a factor for the psychiatric disease schizophrenia.

In 1984, Dante Cicchetti published “The Emergence of Developmental Psychopathology,” an article in which he argued that the previously amorphous study of developmental psychopathology was emerging as a unified discipline. According to Cicchetti, developmental psychopathology describes an interdisciplinary field that studies abnormalities in psychological function that can arise during human development. Such studies include research about the effects that traumatic experiences may have on the development of psychological disorders and about what behaviors are considered normal or abnormal at different ages. In the article, Cicchetti reports about the origins of developmental psychopathology, why it emerged as its own discipline, and why researchers should study it. In addition to recognizing the field of developmental psychopathology, the article informed later research about preventing and treating the harmful psychological effects of early traumatic events.

In the mid-1960s, psychologist John Money encouraged the gender reassignment of David Reimer, who was born a biological male but suffered irreparable damage to his penis as an infant. Born in 1965 as Bruce Reimer, his penis was irreparably damaged during infancy due to a failed circumcision. After encouragement from Money, Reimer’s parents decided to raise Reimer as a girl. Reimer underwent surgery as an infant to construct rudimentary female genitals, and was given female hormones during puberty. During childhood, Reimer was never told he was biologically male and regularly visited Money, who tracked the progress of his gender reassignment. Reimer unknowingly acted as an experimental subject in Money’s controversial investigation, which he called the John/Joan case. The case provided results that were used to justify thousands of sex reassignment surgeries for cases of children with reproductive abnormalities. Despite his upbringing, Reimer rejected the female identity as a young teenager and began living as a male. He suffered severe depression throughout his life, which culminated in his suicide at thirty-eight years old. Reimer, and his public statements about the trauma of his transition, brought attention to gender identity and called into question the sex reassignment of infants and children.