Scientist Franz Max Albert Kramer worked as a psychiatrist in Poland and the Netherlands in the early twentieth century and is known for his contributions to research on psychological conditions that experts call hyperkinetic syndromes. Children with hyperkinetic syndromes display inattention, overactivity, and impulsivity. Along with scientist Hans Pollnow, Kramer defined a specific kind of hyperkinetic syndrome based on an initial case study of seventeen children, initially known as Kramer-Pollnow Syndrome. In 1980, the third edition of the Diagnostic and Statistical Manual of Mental Disorders, or DSM-III, renamed Kramer-Pollnow syndrome to be attention deficit disorder, or ADD. A later revision, in 1987, renamed it attention deficit hyperactivity disorder, or ADHD. Kramer advanced child psychiatry research by laying the groundwork for further research on and understanding of what experts call, as of 2021, ADHD.

Hans Asperger studied mental abnormalities in children in Vienna, Austria, in the early twentieth century. Asperger was one of the early researchers who studied the syndrome that was later named after him, Asperger's Syndrome. Asperger described the syndrome in his 1944 publication Die Autistischen Psychopathen im Kindesalter (Autistic Psychopathy in Childhood). At that time, the syndrome was called autistic psychopathy, and Asperger noted that characteristics of the syndrome included lack of sympathy, one-sided conversations, and difficulty forming friendships. Asperger's work led to the recognition of Asperger's Syndrome as a disorder that results from abnormal development, and the syndrome was later classed on the autism spectrum.

Emil Kraepelin was a physician who studied people with mental illness in the late nineteenth and early twentieth centuries in modern-day Germany. Kraepelin's examination and description of the symptoms and outcomes of mental illness formed the basis for his classification of psychiatric disorders into two main groups, dementia praecox, now called schizophrenia, and manic-depressive psychosis, now called bipolar disorder. He was one of the first physicians to suggest that those researching mental illness should gain scientific knowledge only through close observation and description. However, Kraepelin also believed that genetics played a role in the development and course of mental illness and characterized mentally ill people as weak-willed, which some have argued contributed to stigma about mental illnesses that persist today. Although some historians have pointed out issues with Kraepelin’s teachings, Kraepelin helped to establish psychiatry as a clinical science, which prompted future experimental investigations into mental illness.