Although Leo Kanner, an American child psychiatrist, is commonly credited with “discovering” autism in the 1940s, individuals who exhibited symptoms of ASD had long been recognized as evincing atypical development. Before Kanner’s Autistic Disturbances of Affective Contact in 1943, such persons were frequently considered to have an emotional disturbance or intellectual disability (Wing, 1997). The symptoms he identified among his patients represented the core domains we recognize today as characteristic of autism: communication deficits, difficulty with social interaction and forming relationships, and the presence of restricted and repetitive behavior and interests. He used observations of behavioral symptoms as well as parent-reported family, medical, and developmental history to make his classifications. His method was based on clinical presentation and predominantly atheoretical, a departure from the popular psychoanalytic thinking of his era (Blacher & Christensen, 2011).

Kanner coined the term “early infantile autism ” to describe the constellation of symptoms exhibited by the children he studied (Kanner, 1951). With his publication of detailed case studies (Kanner, 1943), he was considered the first to recognize the denoted behavioral phenotype as disparate from childhood psychosis (Blacher & Christensen, 2011). Nevertheless, his first work on the subject did not specify diagnostic criteria in 1956, Kanner and Leon Eisenberger delineated specific symptoms required for classification (Eisenberger & Kanner, 1956). This development of diagnostic criteria based on observations of child clients was unusual for the time period, when criteria for disorders among children were simply modifications of criteria for syndromes seen in adults (Rutter & Schopler, 1988).

In the late 1950s and early 1960s, diagnostic criteria and terminology for autism remained controversial. From Kanner’s original observations, other diagnostic criteria were created but with notable changes. For instance, Polan and Spencer (1959) published the 30-item Checklist of Symptoms of Early Infantile Autism, which included language distortion, social withdrawal, lack of integration in activities, obsessiveness and nervousness, and family characteristics. These criteria aimed to evaluate the “perceptual apparatus” and “psychogenic factors related to deviant styles of interpersonal relatedness” (Ward, 1970).

In 1958, a “Social Psychiatry Research Unit ” was opened due to the British Government’s impending enactment of the Mental Health Act. There, Hans Eysenck and other influential scientists led the charge for experimental psychology, including the use of statistical analyses and behavioral measures. Eysenck was a strong proponent of direct observation rather than psychoanalytic theory, arguing against Freudian speculative impressions (Evans, 2013). In particular, the psychoanalytic theory of the time suggested that autism was a “reaction to an overwhelming inner or outer assault at a vulnerable developmental stage” (Garcia & Sarvis, 1964, p. 530).

This shift in thinking toward an empirical-based study of psychology helped influence Mildred Creak, a British child psychological professional who studied psychopathology in infancy, to unite prominent members of the field to identify specific features of childhood schizophrenia. Her purpose in forming the work group was to help establish quantitative, reliable research of childhood psychopathology, which included creating criteria that all psychiatrists could agree on in order to enable population-based studies (Evans, 2013). The work group’s efforts resulted in a set of nine key features: impairment of emotional relationships with people; unawareness of personal identity; preoccupation with particular objects; resistance to environmental change; abnormal perceptual experience; acute, excessive, illogical anxiety; a lack of or delay in language ability; distortion in motility patterns; and impaired cognitive function that sometimes occurs with savant skills (Creak, 1961).

Another example of criteria appearing in that era was that by Schain and Yannet (1960). Their criteria for autism included children who displayed “an extreme preoccupation with self and unrelatedness to people” and who thus failed to develop relationships with caregivers as expected by age 2 years (p. 561). These authors noted that they might have included cases that other professionals would not consider to have infantile autism but that they had required Kanner’s “common denominator” of difficulty with social relationships. Their criteria, therefore, did not include display of insistence on sameness or similar symptomatology included in Kanner’s original description.

Unlike the others originating at the time, the criteria authored by Ornitz and Ritvo (1968) emphasized perception issues, considering them fundamental to the other problems in autism. Their criteria encompassed symptoms in the areas of perceptual integration, motility patterns, capacity to relate, language, and developmental rate. That same year, an additional conceptualization was published by Rendle-Short and Clancy. Their “screening test” included 14 symptoms, of which a child needed to exhibit half or more per caregiver report, that the authors considered most representative of the essential characteristics of autism (Rendle-Short & Clancy, 1968). These symptoms were: difficulty engaging with other children, acts as though deaf, resists learning, no fear of real dangers, resists routine change, indicates needs by gestures, inappropriate laughing, not cuddly, marked physical overactivity, no eye contact, inappropriate attachment to objects, spins objects, sustained odd play, and standoffish manner. With the continual development of varying definitions, the boundaries of the disorder remained unclear.

Despite a lack of agreement in the field as to what exactly constituted autism, Dr. Victor Lotter published the first paper to give the results of an epidemiological study of autism among a population of children of varying intellectual function in 1966. To meet his criteria and be considered to have autism, a participant had to have a profound lack of affective contact and elaborate repetitive, ritualistic behavior, whereas early age of onset was not included (Feinstein, 2010). Results suggested a prevalence rate of 4.5 per 10,000 (Lotter, 1966).

In the 1960s, the practice of the family physician completing screening for developmental issues including autism was already in place in a less formal variation than today (Fotheringham, 1969). At that time, the physician might compare the child’s development to established milestones (e.g., motor, communication) and gather more in-depth developmental history from caregivers (e.g., age at first concern, significant biological or social events that affected functioning). If the child was school-aged, a sampling of schoolwork or a brief achievement test may be administered. Nonetheless, in Wing and Wing’s “Early Childhood Autism” (1976), contributor Dr. P. H. Connell noted the deficiency of adequate comprehensive assessment measures for diagnosis, not just screening, of autism.

The aforementioned Checklist of Symptoms of Early Infantile Autism by Polan and Spencer (1959) was considered one of the pioneers of standardized autism assessment. This measure required that each respondent endorses or denies the presence of a specific list of symptoms for the child being evaluated (Rotatori, Obiakor, & Bakken, 2011). In 1964, Rimland, inspired by the aforementioned checklist, attempted to translate Kanner’s definition into an empirical rating scale to identify early infantile autism in children up to age 7 years. The Diagnostic Form E–1 (Rimland, 1964b) was a parent-report measure that included 76 questions inquiring about birth history and onset and characteristics of symptoms. The form was subsequently revised to reflect the need for information about children before age 5 years. Thus, the Diagnostic Checklist for Behavior–Disturbed Children, Form E–2 (Rimland, 1964a) included questions about early development (i.e., from birth through age 5 years). The form included characteristics of autism described by Kanner and symptoms of childhood schizophrenia described by experts in that field. According to Rimland, 31 children had been diagnosed by Kanner prior to their completion of Form E-2, and the E-2 scores correlated strongly with these diagnoses (Rimland, 1971). In addition, he found that the parent-reported presence of “autistic speech symptoms” among children with and without classification of early infantile autism, indicating language issues alone, was insufficient for an autism diagnosis (Rimland, 1971). Although reliability of parent-report measures had been questioned in terms of reliability and accuracy, Rimland argued that diagnosis should require retrospective information, making caregiver report a necessity (1971). To further justify his use of parent report rather than direct observation, he also suggested that behaviors may differ within and outside of the diagnostic session (Rimland, 1971).

Soon after the publication of Rimland’s checklist, Ruttenberg and colleagues published the Behavior Rating Instrument for Autistic and Atypical Children (Ruttenberg, Dratman, Fraknoi, & Wenar, 1966). Reportedly this measure was unable to accurately differentiate between autism and intellectual disability (Parks, 1983). Despite this, teachers and therapists found the measure useful for goal formulation and in predicting future development among children exhibiting atypical development (Feinstein, 2010).

Both Creak’s (1961) criteria and Rimland’s (1964b) original checklist lacked consideration of symptoms among very young children. To correct for this exclusion, Reichler and Schopler developed a 15-scale rating system, initially named the Childhood Psychosis Rating Scale (CPRS) , in 1971. Their aim was to incorporate Kanner’s original description, less common characteristics of autism noted by Creak, and symptoms of autism common in younger children. The observational scale required each of the 15 included domains to be considered in terms of atypicality, frequency, and duration and given a corresponding rating from 1, which represented behavior within normal limits, to 4, which represented severely abnormal behavior (Schopler, Reichler, DeVellis, & Daly, 1980). This measure was later renamed the Childhood Autism Rating Scale (CARS) . An updated version of the CARS is commonly used for assessment at present.

Kanner’s purpose for his descriptions of autism as a syndrome was to recognize a constellation of certain behaviors that differed from symptoms of other mental health issues (Rutter & Schopler, 2012). To clarify the definition, work was needed to establish which symptoms could potentially occur in autism and which were characteristic of autism and therefore requisite behaviors for such a classification. Renowned psychiatrist Sir Michael Rutter was extremely influential in this endeavor. He found three primary types of symptoms evinced by almost all children with autism and that occurred much less frequently among children with other disorders. These symptoms confirmed Kanner’s work and are the same as those core domains we recognize in the field today: difficulty developing and maintaining social relationships, problems with language development and use, and ritualistic or compulsive behavior (Rutter, 1970, 1971). Additional symptoms that occurred frequently among children identified as having autism included stereotypy (e.g., repetitive motor movements), self-injury, poor attention span, and delayed bowel control (Rutter, 1970, 1971).

Most researchers in Britain, Australia, Canada, and the United States supported autism and childhood schizophrenia as discrete syndromes by the 1970s (Green et al., 1984). In 1971, DeMyer and colleagues made an empirical comparison of five diagnostic systems for differential diagnosis between the disorders. The diagnostic systems they used included Polan and Spencer (1959), Rimland (1964b), Lotter (1966), Rendle-Short and Clancy (1968), and Creak/British Working Party (1964). The authors administered all of the checklists to each of the 44 participants. Results indicated overlap of only 35 % across all five systems, reflecting the great disparity in definitions of schizophrenia and autism that existed in that era despite the recognition that the disorders were distinct. Furthermore, the authors noted that all of the checklists lacked rigorous validity studies at the time the study was conducted and, as such, could only serve as screening instruments of relatively equal value (DeMyer, Churchill, Pontius, & Gilkey, 1971). That is, any one of the checklists studied could differentiate early schizophrenic and autistic children from nonpsychotic children, but not necessarily to differentiate within the “psychotic” group. To excuse the low amount of overlap, DeMyer and colleagues pointed out that professionals in close collaboration (e.g., working at the same facility) are much more likely to experience agreement on diagnosis than those experts who do not engage in constant feedback and comparison of diagnoses, despite the use of standardized or structured assessment instruments.