Also called: Social Security tax hump
An income range in which each extra dollar of income also makes up to 85 cents more Social Security taxable, so the real marginal rate is up to 1.85 times the bracket rate.
In the 22% bracket, a dollar that drags 85 cents of benefits into taxable income is taxed at 22% x 1.85 = 40.7%. The effect ends once 85% of benefits are already taxable.
Other income-tested rules can stack on the same dollars: capital gains leaving the 0% rate, the senior deduction phasing out, and Medicare surcharges. RetIQ's Marginal Rate Curve plots the combined effect.