@Agathocles is this FAQ from the IRS hanging you up? this FAQ is referring to what does not get included on Line 8 of Form 8960. Turbo Tax does not include SS benefits on line 8 per the IRS directions.
9. What are some common types of income that are not Net Investment Income?
Wages, unemployment compensation; operating income from a nonpassive business, Social Security Benefits, alimony, tax-exempt interest, self-employment income, Alaska Permanent Fund Dividends (see Rev. Rul. 90-56, 1990-2 CB 102) and distributions from certain Qualified Plans (those described in sections 401(a), 403(a), 403(b), 408, 408A or 457(b)).
