Most states declare a specific policy favoring an ongoing, healthy relationship between the child and both parents. If one parent is trying to undermine the child’s relationship with the other parent, that is a negative factor against the parent who is trying to hurt the relationship. If other factors are close to equal, a court may grant custody to the parent who is more likely to encourage an open and good relationship with the other parent.
Similarly, if a custodial parent regularly interferes with visitation, that is a negative factor against the custodial parent and can lead to modification of custody to the noncustodial parent (assuming the noncustodial parent is able to properly care for the child).
SOURCE: American Bar Association