Imagine logging into your 401(k) after a decade of contributions, expecting to see a portfolio that grew with the market. Instead, your balance is roughly the sum of what you put in. The money never bought a single share of anything.

That scenario is more common than most savers realize. On NerdWallet’s Smart Money Podcast, in the episode How to Put $200K to Work and The Truth About Generational Spending, one host put it plainly: “there could be cash in a brokerage statement that actually isn’t being invested.” The hosts added that “where you get the benefit of the Roth over time is investing it,” not simply funding it. Discovering this years later, they noted, can be “very heartbreaking.”

Quick Read

• A $50,000 401(k) sitting in cash for three years earns ~4% annually while the S&P 500 (SPY) gained 80%, costing uninvested savers nearly $40,000 in compounding wealth.

• This mistake is catastrophic for investors under 40 with decades of compounding ahead, but recoverable for those within five years of retirement who can reallocate immediately.

• A recent study identified one single habit that doubled Americans’ retirement savings and moved retirement from dream, to reality. Read more here.

The cost is bigger than most savers realize

Contributing to a retirement account opens a tax-advantaged container. Picking investments fills it with assets that can grow. Skip the second step and the account is only a holding pen for cash.

Read: Data Shows One Habit Doubles American’s Savings And Boosts Retirement

Most Americans drastically underestimate how much they need to retire and overestimate how prepared they are. But data shows that people with one habit have more than double the savings of those who don’t.

The S&P 500, tracked by the SPDR S&P 500 ETF Trust (NYSEARCA:SPY), returned nearly 80% over the past three years. Over five years, it returned 80%. Over ten years, 260%. A $50,000 401(k) balance fully invested over the last three years would have nearly doubled. The same $50,000 sitting in the plan’s default cash sweep would have earned the cash yield, currently anchored to a Fed funds upper bound of nearly 4%.

Add inflation. The Consumer Price Index sits at 332.4, up 0.6% in a single month. Core PCE, the Fed’s preferred gauge, has climbed from 126.1 in June 2025 to 129.6 in April 2026. Cash earning about 4% in a money market sweep loses ground on a real, purchasing-power basis once taxes and rising prices take their bite.

Why this keeps happening

Most large 401(k) plans auto-enroll you into a qualified default investment, often a target date fund. Older plans, self-directed IRAs, Roth IRAs opened at a brokerage, and some employer plans require manual investment selection. Contributions land in a settlement fund and stay there until you act. Statements showing contributions arriving on schedule feel like progress.

Consumer behavior worsens the problem. The U.S. personal savings rate has fallen from 5.2% in the first quarter of 2025 to 3.7% in the first quarter of 2026. People are putting less aside, so the dollars they do save matter more. University of Michigan consumer sentiment recently dropped to 49.8, a recessionary reading. When people feel anxious, they freeze. Frozen money sits in cash.

The variable that decides whether this is catastrophic or merely costly

The variable is time horizon. If you are 58 and your 401(k) has been sitting in cash for two years, the damage is real but recoverable. The S&P returned about 28% over the past year alone. You can still allocate now and capture future compounding before retirement.

If you are 32 and have been contributing to an unallocated Roth IRA for ten years, the math is brutal. Ten years of missed compounding cannot be replayed. A $6,000 annual contribution from age 22 to 32, invested at a 7% long-term return, would grow to roughly $83,000 by age 32 and could compound to several hundred thousand by age 65 even without further contributions. The same $60,000 in contributions sitting in cash earns a fraction of that and loses purchasing power along the way.

The younger you are, the more urgent the audit.

What to do this week

1. Log into every retirement account you own. Find the “holdings,” “positions,” or “investment elections” tab. If you see “cash,” “money market,” “settlement fund,” or a stable value fund holding more than a small slice of your balance, you have found the problem.

2. Confirm your contribution allocation separately from your existing balance. Some accounts let new cash contributions sit uninvested even after you have selected funds for the existing balance. Both settings need to be active.

3. Choose a default if you do not want to pick individual funds. A target date fund matched to your expected retirement year is the simplest single-decision fix. It rebalances stocks, bonds, and short-term holdings automatically as you age.

4. Reallocate the existing cash balance. Move uninvested dollars into your chosen investments. This is usually a separate transaction from setting future contribution elections.

5. Set a yearly calendar reminder to confirm nothing has reverted to cash after a plan change, rollover, or employer switch. Run the comparison through NerdWallet’s investment calculator to see what a 7% return looks like over five or ten years versus money sitting in cash.

Funding a retirement account is the easy half. The half that builds wealth is making sure the money works.

Data Shows One Habit Doubles American’s Savings And Boosts Retirement

Most Americans drastically underestimate how much they need to retire and overestimate how prepared they are. But data shows that people with one habit have more than double the savings of those who don’t.

And no, it’s got nothing to do with increasing your income, savings, clipping coupons, or even cutting back on your lifestyle. It’s much more straightforward (and powerful) than any of that. Frankly, it’s shocking more people don’t adopt the habit given how easy it is.