What is the mental and physical health history of each parent?
How old is the child, what is the state of the child’s physical/mental health, and (if old enough) does the child have any preference?
What is each parent’s job? What are their personal habits, both good and bad (e.g., clean, healthy, excessive drinking, history of violence)?
Does the child have a particularly strong emotional bond with either parent? How likely is that parent to foster an emotional bond between the child and the other parent?
Will the child have to adjust to a new school, city, quality of life, and friends if living with one parent versus the other?