Divorce decrees can determine who gets to claim the exemption between the custodial parent and the noncustodial parent. Once a child reaches the age of majority, the concept of custody ceases to exist - no one has custody of an adult. For an adult child, the only person who can claim the exemption is the person who qualifies for the exemption under the tax law. If she lives with you for more than 1/2 the year (a student away at school counts as living at their permanent residence), you provided more than 1/2 of her support are the requirements in a nutshell. Since the adult child neither lived with her mother ?nor did her mother provide any support, the mother doesn't qualify - regardless of what the decree says.
Answered on Aug 07th, 2015 at 4:44 AM