Your ability to maintain continuity in the child’s life is one of the strongest arguments you have for maintaining custody if you already have it. A whole body of law has developed around determining what is an “established custodial environment” (ECE).
What the courts are trying to do is minimize the disruption of the child’s life. The divorce is thought to be traumatic enough for the kids without them also having to leave their home, friends, neighborhood and school. If a strong parent-child relationship has formed, courts are loath to break it. Some states have gone so far as to define an established custodial environment by statute. For example, Michigan’s Child Custody Act states: “An established custodial environment is created if, over an appreciable period of time, the child naturally looks to the custodian in such an environment for guidance, discipline, the necessities of life and parental comfort.”
The time element varies. For very young children an ECE can be created in only a few months because they bond much more quickly. For a teenager it can take much longer. What this means is that judges do not like to tinker with custody if the current situation seems to be working. If a child is settled, safely and happily, why uproot him or her? Thus, it is very important, if you want custody, to stay with the children no matter what. You do not want your spouse establishing the custodial environment.