The fight over medical treatment for transgender youth is upending a decades-old consensus that mitigated legal conflicts between states and helped dramatically reduce parental kidnappings during divorce and child custody disputes.

As recently as the 1970s, a mother or father who feared that they were going to lose a custody battle would take the kids and move to another state with more favorable laws. These parental kidnappings were unfair to the other parent, psychologically damaging to the children and all too often successful as a legal tactic.