Americans typically work at seven different companies during their career, and most of them have something to show for each stop along the way.
"They end up with these small 401(k)s or other employer-sponsored accounts that they've left behind," said John F. Sweeney, Fidelity Investments' executive vice president.
New research on the contents of Americans' IRAs and 401(k)s suggests that these orphaned retirement plans often languish untouched since the last automatic-deposit contribution, like dusty museums of our financial needs at the time, and out of whack with our current age and attitude toward investing.
The clues come in a study by the Employee Benefit Research Institute, whose researchers looked at Federal Reserve data on the asset allocations of Americans' retirement accounts.
They found that workers who have more than one kind of retirement investment generally keep a higher percentage of their funds in stocks than those who have one kind of account. More specifically, those who own an IRA are more likely to be all in stocks if they have a 401(k) as well.
(Read More: How to Really Make Americans Save for Retirement)
That lack of balance with more stable, interest-bearing assets, like bonds, suggests that these stock-heavy IRAs are predominantly old 401(k)s that were rolled over for safekeeping as workers left a job and never updated.
"There's a lot of odd investing behavior," said Craig Copeland, an EBRI researcher and author of the study, "but that may be the reason. They'll pay attention to the new 401(k), but just leave the older one."
"It's inertia," agreed Rebecca Hall, a wealth planner with Ameriprise Financial in Reston, Va. "They may never have changed from when they were hired 15 years before."
The phenomenon gives "set it and forget it" a whole new meaning.