If you live in a community Property State, most income you earn is split evenly between you and your spouse.
If there is income relating to something before marriage, such as a rental purchased before marriage, that income might belong only to the owner of the property.
Self-Employed taxpayers split the net income, but not the Self-Employment tax.
Different states have different rules.
Here is a link from TurboTax
Here is a link from the IRS
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