Sexual science or sexology arose in the last three decades of the 19th century when psychiatrists and neurologists began to study and treat deviant sexualities as sickly “perversions.” The new science of experimental psychology did not engage with this morally contested subject. Research into sexuality was rooted in a biomedical and clinical approach. All the same, in the late 19th and early 20th centuries, some medical experts increasingly explained perversion as well as regular sexuality in a psychological way. This trend was intertwined with the changing definition of sexuality as either a pushing or a pulling force, which pertained not only to biological versus psychological interpretations, but also to the contrast between nature and culture, male and female sexuality, and pessimistic and optimistic evaluations. All of this has contributed to the shaping of the modern concept and experience of sexuality and also to its sociopolitical regulation in the 20th-century Western world.
Two different but related developments played an important role in the history of psychologists in the fields of mental health care in Germany during the 20th century. The first development took place in the field of applied psychology, which saw psychological professionals perform mental testing, engage in counseling and increasingly, in psychotherapy in practical contexts. This process slowly began in the first decades of the 20th century and included approaches from different schools of psychotherapy. The second relevant development was the emergence of clinical psychology as an academic sub-discipline of psychology. Having become institutionalized in psychology departments at German universities during the 1960s and 1970s, clinical psychology often defines itself as a natural science and almost exclusively focuses on cognitive-behavioral approaches. There are four phases of the growing relationship between psychology and psychotherapy in Germany in which the two developments were increasingly linked: first, the entry of psychology into psychiatric and psychotherapeutic fields from approximately 1900 until 1945; second, the rise of psychological psychotherapy and the emergence of clinical psychology after World War II until 1972, when the diploma-regulations in West Germany were revised; third, a phase of consolidation and diversification from 1973 until the pivotal psychotherapy law of 1999; and fourth, the shifting equilibrium as established profession and discipline up to the reform of the psychotherapy law in 2019. Overall, the emergence of psychological psychotherapy has not one single trajectory but rather multiple origins in the different and competing academic and professional fields of mental health care.