There is no one factor that is invariably “the” most important factor in a custody case. The importance of a particular factor will vary with the facts of each case. If one parent in a custody dispute has a major problem with alcoholism or mental illness or has abused the child, that of course could be the deciding factor.
If neither parent has engaged in unusually bad conduct, the most important factor often is which parent has been primarily responsible for taking care of the child on a day-to-day basis. Some states refer to this as the primary caretaker factor. If one parent can show that he or she took care of the child most of the time, that parent usually will be favored for custody, particularly if the child is young (under approximately eight years old).