Each dependent that you claim on your federal income taxes reduces your taxable income for the year, which in turn reduces your tax bill. However, the Internal Revenue Service requires that the person you want to claim meet the requirements to be considered a qualifying child or a qualifying relative. To qualify as either one, the child generally can't file a joint return with a spouse.
Joint Return Test
A married child won't meet the requirements to be a qualifying child or qualifying relative unless the child doesn't file a joint return or, if filing a joint return, only does so to get a refund of income taxes withheld or estimated tax paid. For example, if your son and his spouse file a joint return because one or both of them had money withheld from their paychecks, but did not make enough to be required to file a return or owe any income taxes, you could still claim your son -- and even his wife -- if they meet all the other tests. However, if the couple files to claim a refundable tax credit, you can't claim him. Even if you meet this test, you must still meet all the other requirements to claim the child. To be a qualifying child, he must live with you for more than half the year and can't provide more than half his own support. To be a qualifying relative, he doesn't have to live with you, but can't have more than $3,950 in income as of the 2014 tax year and you must provide more than half his support.
Determining Marital Status
A person's marital status is determined as of the end of the calendar year. So, if your child gets married in late December, she is treated as being married for the entire year, and therefore can file a joint return with her spouse. If she does so and doesn't meet the exception of filing just to receive a refund, you can't claim her as a dependent, even if she lived with you for 11 months of the year.