20 CFR 219.40: Evidence of relationship by equitable adoption—child.

(a) Preferred evidence. If the claimant is a person who claims to be the equitably adopted child of the employee (or of the employee's wife, widow, widower, or husband), as defined in part 222 of this chapter, the Board will ask for evidence of the agreement to adopt if it is in writing. The Board will also ask for written statements from the child's natural parents as well as adopting parents concerning the child's relationship to the adopting parents.

(b) Other evidence. If the agreement to adopt was not in writing, the Board will require other convincing evidence about the child's relationship to the adopting parents.