Early Days
Cartesian Dualism: Rene Descartes was the first person in Western Medical history to separate the mind from the body.
"Psychosomatic Illness": The term was developed by Johann Christian August Heinroth.
Psychoanalytic Theory: Sigmund Freud.
Pscyhoanaysis began to be used in hospitals more formally due to the influences of Alan Gregg, Franz Alexander, Stanley Cobb, and Felix Deutsch.
"Liaison Psychiatry": The term was created by Edward Billings. He published A Handbook of Elementary Psychobiology and Psychiatry. Psychosomatic Medicine was published in the same period by Edward Weiss and O Spurgeon English. These two books outlined the theoretical foundations for the field.
"Biopsychosocial Model": The term was stated by George L. Engel, who used the term to challenge divisions created by Cartesian Dualism.

Specific liaison psychiatry services were set up in the 1970's. In 1997 liaison psychiatry was recognized as a distinct Royal College of Psychiatry Faculty. The need for liaison psychiatry is clear due to the high prevalence of psychiatric disorders within a general hospital. In fact, the prevalence of psychiatric disorders for patients in general hospitals is much higher than the general population[1]. The prevalence ranges from 41.3% to 46.5%[2]. When a specific liaison psychiatry service is not present, a significant proportion of psychiatric co-morbidity remains undetected and untreated in hospital [3]. In addition, advancements in medical technology such as intensive care treatment, organ transplantation and cardiac surgery[2] have increased the workload for liaison psychiatry. - edited from writing by Dr. Michelle Tempest

The Faculty of Liaison Psychiatry was established with the Royal College of Psychiatrists.

The American Psychiatric Association formally recognized Consult-Liaison Psychiatry as a subspecialty, with its own sub-specialty board exam. The profession decided that the best term for the specialty would be "Psychosomatic Medicine".

Emotions and Disease: A History from the National Library of Medicine