26 C.F.R. § 1.1402(c)-1
In order for an individual to have net earnings from self-employment, he must carry on a trade or business, either as an individual or as a member of a partnership. Except for the exclusions discussed in §§ 1.1402(c)-2 to 1.1402(c)-7 , inclusive, the term "trade or business", for the purpose of the tax on self-employment income, shall have the same meaning as when used in section 162. An individual engaged in one of the excluded activities specified in such sections of the regulations may also be engaged in carrying on activities which constitute a trade or business for purposes of the tax on self-employment income. Whether or not he is also engaged in carrying on a trade or business will be dependent upon all of the facts and circumstances in the particular case. An individual who is a crew leader, as defined in section 3121(o) (see such section and the regulations thereunder in part 31 of this chapter (Employment Tax Regulations)), is considered to be engaged in carrying on a trade or business with respect to services performed by him after 1956 in furnishing individuals to perform agricultural labor for another person or services performed by him after 1956 as a member of the crew.
26 C.F.R. §1.1402(c)-1