Quick search by citation:

5 U.S. Code § 8420 - Insurable interest reductions

At the time of retiring under section 8412, 8413, or 8414, an employee or Member who is found to be in good health by the Office may elect to have such employee’s or Member’s annuity (as computed under section 8415) reduced under paragraph (2) in order to provide an annuity under section 8444 for an individual having an insurable interest in the employee or Member. Such individual shall be designated by the employee or Member in writing.
The annuity of the employee or Member making the election is reduced by 10 percent, and by 5 percent for each full 5 years the individual named is younger than the retiring employee or Member, except that the total reduction may not exceed 40 percent.
An annuity which is reduced under this subsection shall, effective the first day of the month following the death of the individual named under this subsection, be recomputed and paid as if the annuity had not been so reduced.
In the case of a married employee or Member, an election under this section on behalf of the spouse may be made only if any right of such spouse to a survivor annuity based on the service of such employee or Member is waived in accordance with section 8416(a).
Paragraph (1) does not apply in the case of an employee or Member if such employee or Member has a former spouse who would become entitled to an annuity under section 8445 as a survivor of such employee or Member.