HERE IS HOW I LEARNED THERE IS “NO SUCH THING” AS AN “EXPERT”. (funny story)
About 40 years ago I was prosecuting an airman who murdered his wife. His name was Mike, but because he used a large butcher knife while she was in the shower, we called him “psycho killer”, after the Hitchcock movie.
The defense had an expert psychiatrist [Let’s just call him Doctor] who was prepared to say that Mike was under “extreme emotional distress” at the time, which would have reduced the crime to a lesser offense (whether that was grossly negligent homicide, murder 3d, or some other homicide I do not recall, but the law was clear: if the jury found “extreme emotional distress” they could not convict on the more serious charges.)
So … Doctor took the stand and the defense attorney walked him through the paces. He testified that in response to two exams (again, I do not recall the names of those “medical” tests), ALL of the answers Mike gave (37 if I recall correctly) were “consistent with”* his suffering, or being under, extreme emotional distress.
I kept count of the questions and answers during direct examination – I always used two pens with different colored inks and made notes to be prepared for when I cross-examined. Well, at some point when the Doctor was testifying I had a flash of inspiration on how to cross examine him. The defense attorney sat down and I got up to question Doctor. I asked only 3 questions.
Me (after the usual “good afternoon” and so on): Doctor, I counted the number of times that you testified that Mike gave answers which you say was “consistent with” extreme emotional distress. Those were the words you used every time, 37 times, right, Doctor, “consistent with extreme emotional distress”, aren’t they? He hesitated … I reminded him we could have the court reporter read it all back, and he said something like “well, yes, that’s right”.
Me: Doctor, the fact that the moon appears yellow in color is “consistent with” the conclusion that the moon is made of cheese, is it not? All of the jurors’ eyebrows went up and one or two of them actually laughed out loud – in a murder case!
He did not answer, so I asked the judge to order him to answer the question, and he had to say “yes”.
He was already “toast”, but I simply said “and we all know, don’t we, Doctor, that the moon is not made of cheese?” I didn’t even wait for an answer. I sat down, and I did not bother to call our own expert psychiatrist. Mike was convicted of 1st degree murder.
So much for “expert” testimony.
Because psychiatry and many, many matters about which “experts” testify is as much art as science, they cannot say “positively”; they must say “consistent with”. Even with DNA they cannot say “positively”, they have to give probabilities. I.e. The probability of anyone else having the same DNA is 1 in 25 gazillion, or similar testimony.
See related articles on this website (Experts on hair samples, fingerprints, etc.) : type in the key word “expert” in the search bar.
Read some articles on this website to realize that today  faulty forensic evidence [experts] contributes to wrongful convictions; nobody knows the real number, but maybe as high as 40% of all wrongful convictions, and there are hundreds each year.