Income and expenses that belong to the taxpayer and spouse are assigned to the taxpayer and spouse.
Items that do not directly belong to one or the other may be allocated on some basis such as income.
So if one earned $50,000 and the other earned $25,000, the first may be given 2/3rds of the credit or deduction and the second would be allocated 1/3rd of the credit or deduction.
Make note of the allocation method that you choose to use and retain that note in your tax files.
Part III on page 3 of the Instructions for IRS form 8379 explains the allocation of taxes, credits and payments.
To properly determine the amount of tax owed and overpayment due to each spouse, an allocation must be made as if each spouse filed a separate tax return instead of a joint return. So, each spouse must allocate their separate wages, self-employment income and expenses (and self-employment tax), and credits, such as education credits, to the spouse who would have shown the item(s) on their separate return.
Other items that may not clearly belong to either spouse (for example, a penalty on early withdrawal of savings from a joint bank account) would be equally divided.
If a deduction or credit would not be allowed had you filed a separate return, use the deduction or credit shown on your joint return and allocate that amount between you and your spouse. An example of a deduction that is generally not allowed on a separate return is the student loan interest deduction. Examples of credits not allowed on a separate return are the child and dependent care credit and the American opportunity credit. A similar rule applies to income and deductions (such as taxable social security benefits and the IRA deduction) that are subject to special limits on a separate return. Use the income and deductions shown on your joint return and allocate them between you and your spouse.
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