FIGURE 1.1 Schizoid Personality as Defined by DSM‐I Through DSM‐5 and the AMPD of DSM‐5

Leading up to the publication of DSM‐5, many researchers pushed for the inclusion of more dimensional representations of psychopathology. A dimensional approach suggests that symptoms and traits exist on continuums. Rather than putting people into yes–no categories based on whether or not they have a certain number of symptoms, researchers who advocate for a dimensional approach place people on a dimension of symptom severity ranging from not present or not severe to very severe, for example. This is in contrast to a categorical representation of psychopathology (the majority of the DSM) where symptoms are assessed and a clinician makes a dichotomous (yes/no) decision about the presence of a diagnosis. Some studies have found that these distinct, diagnostic categories are supported by empirical data, but most studies have found that they are not.

      Subsequently, a number of dimensional components were integrated into the DSM‐5 (Regier, Kuhl, & Kupfer, 2013). For example, an alternative dimensional model for personality disorders was introduced to Section III of the DSM for “Emerging Measures and Models” for future research. Additionally, the diagnoses of autistic disorder, Asperger’s disorder, and pervasive developmental disorder were combined into one autism spectrum disorder. This reflects an understanding that these disorders do not differ in “kind” of symptoms or problems, but in “degree” (of severity). Finally, and importantly to the topic of developmental psychopathology, the DSM‐5 had several revisions that improved the assessment of psychopathology in children and adolescents. Specifically, the DSM‐5 added a heading entitled “Development and Course” to each disorder section to describe the typical development of an individual with that disorder across the lifespan and how the individual might present during each developmental stage. The text of many disorders now also expands upon individual variables or characteristics important to the etiology of that disorder, including culture and gender.

The International Classification of Diseases (ICD)

      While the DSM is the standard diagnostic manual used in the US, the ICD is the classification system of mental disorders used most widely in the world (Reed et al., 2019). Published by the World Health Organization (WHO), the ICD system was developed in order to catalog and track diseases across populations. The primogenitor of the ICD, the International List of Causes of Death, was published in 1883 and revised four times throughout the following half‐century until the newly formed WHO assumed the responsibility for disease classification in 1948 (ICD‐6; Hirsch et al., 2016). Of note, the ICD‐6 was the first edition to include psychiatric disorders in a compilation of diseases that had previously been more traditionally medical.

Until recently, the ICD‐10, published in 1992 and now named the International Statistical Classification of Diseases and Related Health Problems, was the latest iteration of the ICD currently in use. The eleventh iteration of the ICD will come into effect on January 1, 2022 (WHO, 2019) and makes several changes over the ICD‐10 while maintaining the goal of prioritizing clinical utility (Reed et al., 2019). Taxonomically, the boundary between disorders usually associated with childhood and adolescence versus adults was removed, reflecting a similar shift to a lifespan approach that we saw in the DSM. Also, similarly to the DSM‐5, the ICD‐11 includes more dimensional approaches to psychopathology. Dimensional qualifiers have been added to describe the symptom presentation of psychotic disorder, and the conceptualization of personality disorders has been overhauled and resembles the Alternative Model of Personality Disorders (AMPD) found in Section III of the DSM‐5.

Quantitative Classification Approaches

      The histories of the DSM and the ICD are rooted in psychiatry and a largely categorical approach to classification and diagnosis. The ICD and DSM can also be thought of as “top down” approaches because they rely on the authoritative opinion and clinical experience of psychiatrists to organize symptoms or behaviors into groups or categories. However, there have also been individuals who have suggested that “bottom up” approaches to defining types of psychopathology are ideal. “Bottom up” approaches to the classification of psychopathology often take a statistical or factor analytic approach to organizing symptoms. One of the first articles using “bottom up” analyses to investigate the statistical covariation of symptoms was by Moore (1930). More recently, the well‐known Achenbach System of Empirically Based Assessment (ASEBA), the Research Domain Criteria (RDoC), and the Hierarchical Taxonomy of Psychopathology (HiTOP) have been developed.

      The Achenbach Sysyem of Empirically Based Assessment (ASEBA)

The ASEBA was developed by Dr. Thomas Achenbach in the 1960s in order to provide clinicians with a tool for assessing psychopathology in children and adolescents (Achenbach, Rescorla, & Maruish, 2004). At that time, the DSM provided very little information about mental illness in childhood. To develop the ASEBA, he first developed self‐report questionnaires that asked about all types of psychopathological symptoms. Using factor analysis, Dr. Achenbach was able to determine which symptoms co‐occurred with one another and seemed to “hang together.” This allowed him to identify different psychological syndromes, similar to how groups of symptoms are listed under a disorder in the DSM. Today, when a clinician or researcher uses one of the measures of the ASEBA, they can use computer software to score the questionnaire and create a symptom profile displaying their score on each syndrome. Using norms established by studying large pools of people of the same age and gender, these profiles indicate how severe a person’s symptoms are compared to others like them. The ASEBA is a “bottom up” approach to understanding and classifying psychopathology and the scales are dimensional because they do not create distinct categories (e.g., those with depression and those without).

      The Research Domain Criteria (RDoC) Initiative

In 2009, the National Institute of Mental Health (NIMH) launched the RDoC initiative (Insel et al., 2010). This was part of the Institute’s strategic plan to begin developing a dimensional system of psychopathology, ultimately aiming to revamp the traditional categorical models used in the field (i.e., DSM and ICD). The RDoC initiative is not a classification system per se. It is a systematic framework or template for guiding and conducting psychopathological research from a “bottom up” dimensional approach (Kozak & Cuthbert, 2016). Thus, much of the research conducted as part of RDoC examines transdiagnostic symptoms or