The Kumho case stands for the proposition that testimony by expert witnesses must be reliable and grounded in the scientific method (i.e., something is scientifically provable when repeated testing of the hypothesis reaches the same result).
In Kumho, the Supreme Court found that the expert came to his conclusion based upon a scientific approach requiring proof of four factors, even though only TWO of the factors were proven. The court determined that (and assuming that the test was scientific to begin with) that the failure of the expert to satisfy all of the factors of his own test, that the experiment failed to demonstrate the conclusion that the expert reached in his testimony.
How does this apply to rebutting a custodial evaluation? This is my shot at the question, and I haven't spent much time thinking about it.
Does the expert have a methodology that is followed? What is it?
Is the method scientific (will it reach the same conclusion if used repeatedly)?
Did the expert follow the method in reaching the conclusion testified to?
Is the conclusion reached the objective result of the test?
If any one of the above are false, then the expert's testimony isn't admissible, because it devolves into nothing more than lay opinion devoid of any science.
So, given the evaluation of a social worker, how many reports to the court do you think will satisfy the four factors? Damn few.
For example, in an evaluation that examines the parent-child relationship, what factors does the examiner observe and note, and based on those factors, what would be the conclusion taking away the existing circumstances and applying them to any random set of parents and children. Is the conclusion based on science, or is it merely based on the observer's gut instincts?
If it's based on science, it's admissible. If it's not, then it's not!
Winning something like this, would require a considerable amount of analysis of the report, and some very specific questions posed to the expert.
The biggest hurdle here may be the fact that many states do not follow the federal rules of evidence, and may not adopt the Kumho standard as part of their domestic relations law. This is not a constitutional issue so states are free to adopt some other standard as long is it's not "unreasonable."
However, you could certainly try to argue that the state court SHOULD adopt the Kumho standard, and if you win, then they will have adopted it.
One more thing. Making this argument without an attorney is not going to be easy. You're going to have to really hold the court's attention to get all of this stuff into evidence, and you will definitely have to be prepared to appeal on the evidence admitted.
And...if the court refuses to consider your evidence, then the appeals court, even if it goes along with your reasoning, will simply send the issue back to the trial court for consideration of your evidence, which of course will be completely different, because a year or more will have passed, and the child and parents will have moved on in their lives.
All of which may put you back where you started...which is nowhere. But hey...if you win at trial the first time, then it will be the other parent's problem, so it's definitely worth a shot.
One last issue is worth considering: If you manage to exclude the custody eval, then you better have a good legal argument as to why you should be granted substantial custody. Otherwise, you will be right back in the land of judicial bias, because the judge will be ruling based on HIS/HER instincts. And the judge doesn't have to meet the "expert" witness standard of Kumho. How ironic!