Rule 702. Testimony by Expert WitnessesFirst, the expert must be qualified. Having a PhD in psychology is not a qualification unless there is some issue in dispute where the psychologist has specialized training. For example, if the mom is accused of having a borderline personality disorder, and the psychologist is trained at making a DSM-IV diagnosis, then he would be qualified to give an opinion on that. But if he is trained to advise men on coming out of the closet and the court issue involves setting an alarm clock, then he is not qualified.
A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:
(a) the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
(b) the testimony is based on sufficient facts or data;
(c) the testimony is the product of reliable principles and methods; and
(d) the expert has reliably applied the principles and methods to the facts of the case.
Next, the purpose of the testimony is to help the judge understand the evidence, and not to draw conclusions about the outcome of the case. So if the mom says that her manic episodes are controlled by drugs, an expert might explain the practical meaning of that.
The expert testimony should be all about how generally accepted principles and methods apply to the facts of the case. The expert should be explaining, not drawing conclusions or writing orders.
The expert does not necessarily have to determine the facts. If some key fact is under dispute, he can simply state his assumptions, and explain how his analysis would differ depending on whether his assumption is correct. Eg, an expert might say that the blood on the crime scence does not match the defendant, assuming that the samples were collected properly. The defendant is free to argue that the assumption is invalid, in which case it should be obvious how that affects the expert's opinion.
Most states have adopted the federal rule for expert evidence. California has not, and still uses and older rule. The difference is not significant to my points here. I cite the newer rule because it is more clearly written, as it has been refined several times over the last 20 years.
When you get an expert forensic report, the first things to look for are the assumptions, the principles and methods, and the explanations of how those principles and methods were applied to the facts. Everything else in the report is just inadmissible fluff.
I have yet to see a family court report that even satisfies these bare-bones requirements for admissibility as evidence. Have you? For those of you who have had the misfortune of being subjected to one of these reports, what percentage of the report contained admissible evidence?