One may ask how it was that psychology became to be accepted as a specialist field when it came to the law. Thankfully this question has a more-or-less direct answer.
Jenkins v The United States (1962) was the chief case whose outcome was that psychologists offered a unique perspective and whose expert testimony should be included, and considered seriously within a court of law.
The accused in this case was a man who was on trial for assault and intent to rape had raised the defense of schizophrenia. Although the judge initially asked the jury to disregard the 'opinion' of the psychologists who had testified, on appeal the decision was overturned. It was at this crucial turning point that clinical psychology became accepted in broad terms of being able to offer an expert opinion.
Links and further info:
https://www.apa.org/about/offices/ogc/amicus/jenkins
https://en.wikipedia.org/wiki/Forensic_psychology
https://www.jstor.org/stable/20782606
the 1960's was also a booming time for psychology. Think of all the Timothy Leary LSD experiments with Ram Dass. Think of what was going on at the time and how it affected everything. Think about the " Self-Help movement." Think about what was going on during that time that effected so much change for everyone and each individual and the psych.
Interesting birth of clinical psychology in terms of forensics. It's really a new field. And now, after 60 years, a number of developments have ensued.