Parental Alienation is where one parent causes the joint children to dislike the other parent. Proof of this is difficult in Court, but if a court, a Guardian Ad Litem or a Custody Evaluator finds one parent is making the other a villain or disparaging the parent to the children, it is likely to be held against the person doing that. It especially pernicious when custody cases are pending because it is done then in order to gain an unfair advantage against the other parent. It is also likely to come back to haunt the alienating parent later, when the child sees what happened as an adult or teen looking back. It also causes the case to be more involved, more likely to require a credentialed mental health provider to act as an expert and therefore more expensive.