What if I was more responsible for the child(ren) than my spouse?
If you looked after the children more than your spouse did, you are considered the "primary caregiver." The judge will look at which parent prepares meals for the child(ren), gets them ready for school or bed, helps with homework, schedules medical appointments, drives the child(ren) to activities, provides discipline for the children, etc. The judge will generally favor the primary caregiver when considering who to declare the custodial parent.
Judges generally feel that by awarding the child(ren) to the primary caregiver they are preserving the emotional and physical bonds that have developed between the parent and the child. The primary caregiver can continue to provide for the child(ren) without too much disruption, and the child(ren) will stand a better chance of getting through the emotional upheaval of the divorce or separation.
As a practical matter, the doctrine of the primary caregiver often results in favoring the mother over the father. In our society, it is still the mother who typically stays home with the children or assumes the bulk of the childcare. The majority of children end up in the care of the mother not because of a preference for mothers, but because of the way Western society is structured. Be prepared to demonstrate to the judge’s satisfaction that you are the primary caregiver if you are claiming such.