Since 1984, Social Security benefits have been partially subject to federal income tax. The share of benefits subject to tax depends on an expanded measure of income: adjusted gross income plus tax-exempt bond interest and one-half of Social Security benefits. Benefits are only subject to tax if this expanded income measure exceeds $25,000 (single) or $32,000 (married filing jointly). Above these thresholds, up to 50 percent of benefits are included in taxable income if the income measure is below $34,000 for singles or $44,000 for joint filers. For those with higher incomes, legislation enacted in 1993 increased the maximum inclusion rate to 85 percent of benefits; the 85 percent factor was intended to approximate the tax treatment of private pensions.