Parental alienation is a process by which one parent psychologically manipulates a child to the extent that the child becomes estranged from the other parent. There is feverish debate in the psychological community as to how to define it or what the criteria are. For this reason, it is difficult to identify a practitioner as a certified expert in the field, and that means it is difficult to present conclusive evidence of parental alienation in a court of law.
That said, many cite the following dynamics as “proof” of parental alienation: a child’s suddenly and without reason rejecting a parent, or, conversely, a child’s (also without reason) aligning and universally agreeing with a parent.
The best advice, when a parent suspects that their child has been so manipulated, is to seek mental health professionals and legal experts.