Basics of the Ner Investment Income Tax. The Net Investment Income Tax (NIIT) is imposed by I.R.C. Section 1411. The NIIT applies at a rate of 3.8 percent to certain net investment income of individuals, estates, and trusts that have income above the statutory threshold amounts. The NIIT went into effect on January 1, 2013. [ IRC § 1411 , added by , §1402(a), Pub L No 111-152 (Mar 30, 2010).] (See Chapter 3...