Boomers Control $90 Trillion in the “G-Shaped Economy.” At 70, the Safe Money Is What Makes His Social Security Taxable.
Treasury yields finally rewarding retirees sounds like a win, until the IRS formula that Congress wrote in 1984 and never updated turns that interest income into a stealth tax on Social Security benefits most people never saw coming.
This post may contain links from our sponsors and affiliates, and Flywheel Publishing may receive compensation for actions taken through them.
Picture someone who just turned 70 last month. They waited until now to file for Social Security, so their monthly check is roughly 29% larger than it would have been at their full retirement age of 66 and four months. They also have serious money in the bank, because two years of real Treasury yields made cash worth holding on its own terms. That combination puts them squarely inside what Ed Yardeni has been calling the “G-shaped economy,” where boomers hold nearly $90 trillion, or 52% of U.S. household wealth, including about $3.1 trillion in money-market funds.
Higher rates have been a quiet windfall for this group. Retirees finally earning real interest keep asking online why their tax bill jumped when their Social Security check barely moved. The answer sits in a formula Congress wrote in 1984 and last touched in 1993.
Why Interest Income Silently Taxes Your Benefits
The IRS uses a formula called combined income, sometimes called provisional income, to decide how much of your Social Security is taxable. It is your adjusted gross income (AGI), plus any tax-exempt interest, plus half of your Social Security benefit. Cross $25,000 single or $32,000 joint, and part of the benefit becomes taxable. Cross the second tier of $34,000 single or $44,000 joint, and the taxable share climbs toward a ceiling of 85%. The lower thresholds have stayed frozen since 1984. Congress added the upper thresholds in 1993 and has not touched them since. Wages, benefits, and yields have all moved, which is why each year pulls more retirees across the line.
Here is what today’s yields do to that math. A single retiree with $700,000 in short Treasuries earns about $28,000 a year at the current 52-week yield near 4%. Add half of a $36,000 Social Security benefit, another $18,000, and combined income lands at $46,000, well past the top threshold. Crossing that line starts a phase-in rather than flipping a switch. At $46,000, about $14,700 of the benefit becomes taxable. The pressure comes from what happens next: each additional dollar of interest pulls 85 cents of Social Security into the tax base alongside it, so a dollar earned on a Treasury bill adds $1.85 to taxable income. This filer sits in the 12% bracket, which means that interest is effectively taxed at 22.2%.
Had the same $700,000 earned 1%, it would have produced about $7,000. Combined income would have landed at the first threshold, leaving the benefit untouched. The Social Security benefit is identical in both calculations. The yield does all the work, twice. Scale matters. Assuming no other income, a retiree with $400,000 instead of $700,000 has about $20,500 of income after the taxable-benefit calculation. That falls below the combined $24,150 available from the standard deduction, the age-based addition and the enhanced senior deduction. The formula bites hardest on cash piles large enough to clear that shelter.
How This Collides With the Rest of Retirement
At 70, two other pieces press on the same line. Required minimum distributions (RMDs) from traditional IRAs begin at 73, stacking future taxable income on top of today’s interest. And the reward for waiting feeds the formula directly: a check 29% larger puts a correspondingly larger “half of Social Security” figure into combined income.
A few moves change the answer while leaving the lifestyle intact. Interest earned on assets already inside a Roth IRA stays out of AGI. Converting pretax IRA assets into a Roth raises combined income in the conversion year but can reduce it in later years. Municipal bond interest escapes federal tax and still lands in the combined-income calculation, which catches people who bought it for exactly this reason. Treasury interest escapes state tax, a real edge in high-tax states.
Put Your Cash Beside the Combined-Income Formula
Before renewing the next CD or Treasury ladder, three details deserve a look:
- Run your own combined-income figure. AGI, plus tax-exempt interest, plus half the benefit. One line tells you which side of $34,000 or $44,000 you are standing on.
- Know the marginal cost, not just the bracket. Inside the phase-in, an extra dollar of interest carries 85 cents of benefit with it, so a 12% bracket behaves like 22%.
- Locate the account, not just the yield. The same 4% earns very differently inside a Roth, a taxable brokerage account, or a traditional IRA headed for RMDs at 73.
And spreading Roth conversions across the lower-income years before RMDs begin is often the highest-leverage decision on the table (we sized up that quiet window between your last paycheck and your first RMD in a free Roth conversion guide). Boomers hold half the country’s wealth, and much of it is finally earning something. The yield arrives every month; the formula arrives once a year, and it has been waiting since 1993 for rates to come back.
Contact [email protected] for any questions or corrections.