The works of Edward Kruk, Ph.D define it most comprehensively:
What children of divorce most want and need is to maintain healthy and strong relationships with both of their parents, and to be shielded from their parents’ conflicts. Some parents, however, in an effort to bolster their parental identity, create an expectation that children choose sides. In more extreme situations, they foster the child’s rejection of the other parent. In the most extreme cases, children are manipulated by one parent to hate the other, despite children’s innate desire to love and be loved by both their parents.
Parental alienation involves the “programming” of a child by one parent to denigrate the other “targeted” parent, in an effort to undermine and interfere with the child’s relationship with that parent, and is often a sign of a parent’s inability to separate from the couple conflict and focus on the needs of the child. Such denigration results in the child’s emotional rejection of the targeted parent, and the loss of a capable and loving parent from the life of the child.
Psychiatrist Richard Gardner developed the concept of “parental alienation syndrome” 20 years ago, defining it as, “a disorder that arises primarily in the context of child custody disputes. Its primary manifestation is the child’s campaign of denigration against a parent, a campaign that has no justification. It results from the combination of a programming (brainwashing) parent’s indoctrinations and the child’s own contributions to the vilification of the target parent.” Children’s views of the targeted parent are almost exclusively negative, to the point that the parent is demonized and seen as evil.
Parental alienation involves a set of strategies, including bad-mouthing the other parent, limiting contact with that parent, erasing the other parent from the life and mind of the child (forbidding discussion and pictures of the other parent), forcing the child to reject the other parent, creating the impression that the other parent is dangerous, forcing the child to choose between the parents by means of threats of withdrawal of affection, and belittling and limiting contact with the extended family of the targeted parent.
There is now scholarly consensus that severe alienation is abusive to children (Fidler and Bala, 2010), and it is a largely overlooked form of child abuse (Bernet et al, 2010), as child welfare and divorce practitioners are often unaware of or minimize its extent. As reported by adult children of divorce, the tactics of alienating parents are tantamount to extreme psychological maltreatment of children, including spurning, terrorizing, isolating, corrupting or exploiting, and denying emotional responsiveness (Baker, 2010). For the child, parental alienation is a serious mental condition, based on a false belief that the alienated parent is a dangerous and unworthy parent. The severe effects of parental alienation on children are well-documented; low self esteem and self-hatred, lack of trust, depression, and substance abuse and other forms of addiction are widespread, as children lose the capacity to give and accept love from a parent. Self-hatred is particularly disturbing among affected children, as children internalize the hatred targeted toward the alienated parent, are led to believe that the alienated parent did not love or want them, and experience severe guilt related to betraying the alienated parent. Their depression is rooted is feelings of being unloved by one of their parents, and from separation from that parent, while being denied the opportunity to mourn the loss of the parent, or to even talk about the parent. Alienated children typically have conflicted or distant relationships with the alienating parent also, and are at high risk of becoming alienated from their own children; Baker reports that fully half of the respondents in her study of adult children who had experienced alienation as children were alienated from their own children.