Draft Form 1040 instructions. Page 16 has a chart for qualifying a person as a dependent, and the rules run through page 19, including this:
Qualifying child of more than one person.
Even if a child meets the conditions to be the qualifying child of more than one person, only one person can claim the child as a qualifying child for all of the following tax benefits, unless the special rule for Children of divorced or separated parents, described earlier, applies.
- Dependency exemption (line 6c).
- Child tax credits (lines 52 and 67).
- Head of household filing status (line 4).
- Credit for child and dependent care expenses (line 49).
- Exclusion for dependent care benefits (Form 2441, Part III).
- Earned income credit (lines 66a and 66b).
Child Adoption Qualifying child of more than one person
No other person can take any of the six tax benefits just listed unless he or she has a different qualifying child. If you and any other person can claim the child as a qualifying child, the following rules apply.
If only one of the persons is the child’s parent, the child is treated as the qualifying child of the parent.