In a child custody proceeding, the court will make its determination of custody issues based on the "best interest of the child."  The best interest of the child is determined on a case-by-case basis by weighing several factors listed in the Illinois Marriage and Dissolution of Marriage Act (IMDMA) as well as any other factors that the court finds relevant, including those that have been developed by case law. 

‍The factors listed in the IMDMA are:

• The wishes of the parents; 
• The wishes of the child; 
• The interactions and the relationships between the parents and the child; 
• The child's adjustment to his or her home, school, and community; 
• The mental and physical health of both the parents and the child; 
• Whether abuse has occurred either to the child or to any other person; 
• Whether the parents have the ability to cooperate; 
• The living situation of the parents; and
• Whether either of the parents is a sex offender.

In addition to these statutory factors, the court may consider any other facts or factors relevant to the "best interest of the child."  The following factors have been found relevant by previous case law:

• The race & religion of the parents and the child; 
• The parents' work schedules and availability; 
• One parent's interference with the other parent's relationship with the child; 
• Whether one parent has made false allegations of abuse; 
• Whether either parent is living with someone of the opposite sex; 
• Parents' substance abuse; 
• Whether the parent seeking custody is a natural parent.

‍The "best interests of the child" is a fact specific inquiry in which the court has very broad discretion.  Because of the broad discretion of the trial court in these matters, appeals of the trial court's decision are rarely heard. ​

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