The net investment income tax is: 0.9% of the amount by which the taxpayer's modified adjusted gross income exceeds the threshold. 0.9% of the lesser of the taxpayer's investment income or the amount by which the taxpayer's modified adjusted gross income exceeds the threshold. 3.8% of the amount by which the taxpayer's modified adjusted gross income exceeds the threshold. 3.8% of the lesser of the taxpayer's investment income or the amount by which the taxpayer's modified adjusted gross income exceeds the threshold.