Impact on Individuals
For individuals, the effect can be even more severe, because the taxation of Social Security benefits potentially overlaps not just the 15% tax bracket, but the 25% tax bracket (which starts at "only" $36,250 for individuals). As a result, individuals could face a potential tax rate boost from 25% to 46.25%!
The Example of Harry
Harry is an individual with $36,000 of income but a hefty $22,000/year of Social Security benefits. His Social Security provisional income is $36,000 + $11,000 = $47,000, which is $13,000 over the upper threshold for individuals. As a result, $15,550 of his Social Security benefits are subject to taxation (which is 50% of the amount from $25,000 to $34,000, plus 85% of the excess of provisional income above the $34,000 threshold), which puts his AGI at $51,550. Even after a standard deduction and one personal exemption, Harry's taxable income would be $51,550 - $6,100 - $3,900 = $41,550, which places him in the 25% tax bracket.
If Harry now takes an additional $1,000 from his IRA, his provisional income increases to $48,000, his taxable Social Security benefits increase to $16,400, and his AGI rises to $53,400. The net result: Harry's AGI increased by $1,850 for "just" a $1,000 IRA withdrawal, and with a 25% tax bracket his liability will be $1,850 x 25% = $462.50, which equates to a whopping $462.50 / $1,000 = 46.25% marginal tax rate!
The Bottom Line
In reality, it takes a unique combination of income and Social Security benefits to face this 46.25% marginal tax rate, although it is possible; due to how the tax brackets line up with the Social Security taxability thresholds, it's more common for individuals than married couples (the latter generally don't see this 46.25% marginal tax rate unless they are slowly phasing in a very large pool of combined Social Security benefits).
Because the tax brackets are indexed for inflation while Social Security benefits are not, the overlap of 85% Social Security benefits inclusion and the 25% tax bracket is actually declining over time (as the 10% and 15% brackets widen slightly each year). Nonetheless, the fact remains that the phase-in of Social Security benefits can result in surprisingly high marginal tax rates for someone with relatively "modest" income levels - as even a 15% tax bracket can be boosted up to 27.75%!