26 U.S. Code § 6017 - Self-employment tax returns
Every individual (other than a nonresident alien individual) having net earnings from self-employment of $400 or more for the taxable year shall make a return with respect to the self-employment tax imposed by chapter 2. In the case of a husband and wife filing a joint return under section 6013, the tax imposed by chapter 2 shall not be computed on the aggregate income but shall be the sum of the taxes computed under such chapter on the separate self-employment income of each spouse.
Source(Aug. 16, 1954, ch. 736, 68A Stat. 739.)