Professional negligence cases against psychologists involve the same factors as all other negligence cases. You would be looking for a medical malpractice attorney to assist you with this as they are best suited to handle these types of claims even if psychologists are not required to have a medical degree and they do not prescribe medication.
Trying to find one may be tricky if the perception is that the psychologist is broke or there is no malpractice insurance policy in place. Also, a lawyer would need to be able to readily find another such expert to testify against the one that is at issue.
The main issue may be that of immunity protecting expert witnesses from liability. Most states follow the common law doctrine that provides absolute immunity in witness testimony. The U.S. Supreme Court had weighed into the matter in Briscoe v. Lahue, 460 U.S. 325 (1983), declaring that fact witnesses in criminal proceedings have immunity. The principle was expanded over time to cover expert witness testimony as well. Now a few states have created an exception and NY does not appear to be one of them as of yet if the expert was a friendly to your side in the proceedings.
I know this is not the answer you wanted to hear, but I am assuming that you are paying for a professional and honest answer.
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