"In the best interest of the child" or "child's best interest" is the famous mantra of the family court prevalent in child custody proceedings today, yet its interpretation by the family court or judges is often arbitrary and its meaning is still obscure. Moreover, the law regarding child custody varies from state to state, leaving no uniform legal position regarding what is in the best interest of the child. Some states have a preference and presumption towards joint custody, while others do not. Some states are amending laws to adopt a preference and presumption for joint custody, while others are amending laws to allow joint custody only when the parents agree to it.

Like most states, the standard for child custody determination in California is the overall best interest of the child such that it assures the "health, safety, and welfare" of the child and "frequent and continuing contact" with both parents. However, California does not establish a preference or a presumption for or against joint custody or custody to one parent, and therefore leaves the parenting plan decision up to the discretion of the family court or judge. In 1979, California adopted a presumption for joint custody, but later amended the law in 1994 to allow joint custody only when the parents agreed to it. According to the American Bar Association website, other states such as Connecticut, Maine, Michigan, Mississippi, Nevada, Vermont, and Washington also adopted laws in favor of joint custody, but only when the parents agreed to it. Other states, such as the District of Columbia, Florida, Idaho, Iowa, Kansas, Louisiana, Minnesota, Missouri, Montana, New Hampshire, New Mexico, and Texas, have laws favoring a presumption for joint custody. In a few other states, joint custody is not specifically authorized.

Regardless of each state's position for or against a presumption or preference in favor of joint custody and whether or not it has been specifically authorized, overall there appears to be a growing trend in favor of joint custody and more and more bills are being introduced to adopt a presumption that joint custody is in the best interest of the child unless certain circumstances apply (such as convincing evidence that a parent is unfit or that it would not be in the best interest of the child to award joint custody).