The biggest problem that many of today’s retirees have may be spending their life savings—or even spending at all.

A recent study conducted by the BlackRock Retirement Institute and the Employee Benefit Retirement Institute found that most retirees have 80 percent of their pre-retirement savings nearly two decades after they quit work.

The survey was based on a sample of 7,148 households in terms of assets and a sub-sample of 1,660 retirees households on the expenditure side. It was segmented into three groups: the lowest wealth group with zero to $200,000 in pre-retirement, non-housing assets; the middle group with $200,000 to $500,000; and those with $500,000 or more. So these folks were not retired CEOs and investment bankers.

The findings challenge conventional wisdom among advisors and widely respected academic theories about lifecycle consumption developed by Nobel Laureate Franco Modigliani. In fact, more than one-third of the retirees in all three wealth groups grew their assets in retirement.

What explains this behavior? One thing is clear—many retirees either didn’t need to or didn’t want to spend their savings. But it’s not clear how much of their thriftiness can be attributed to parsimonious behavior and what is explained by other factors.

Anne Ackerley, managing director and head of BlackRock’s defined contribution business, has a few ideas, including the possibility that many of these retirees may be afraid of outliving their money.

Ackerley suspects this is a major factor; she added that less than 18 percent are actually overspending and that awareness of longevity risk is on the rise.

This is a serious question. Anecdotal evidence from financial advisors reveals that they tend to have two kinds of retired clients: a small group who overspend and a much larger group who live well within their means. Among this group, most were natural-born savers accustomed for years to seeing their life savings appreciate. When the monthly paychecks stop coming in, it influences their spending.

Second, since the majority are folks who saved all their lives, many may not “know how to spend” once they enter the de-cumulation phase of their lives, she said. That's one of the reasons why BlackRock is developing tools, sort of target date funds in reverse, to help retirees manage spending in a way that doesn’t cost them sleep.

One of the smartest advisors I know, a specialist in financial life planning, told me when he retired at 75, “it’s scary.” And I would guess by any standards outside of Wall Street or Silicon Valley, this advisor is not only an authority, but very well-off.

But there’s also a third possibility. As Ackerley says, maybe today’s retirees don’t have to spend their retirement assets because they are among the “lucky few.”

Many of today’s retirees in their seventies enjoyed both a pension and a 401(k) plan, which for them was a supplemental savings option, not their primary accumulation vehicle. One suspects a healthy chunk of these folks also have dividend-paying stocks.

Public policies have also tilted in their favor. Retirees also are benefitting from changes to Social Security in the 1980s and Medicare in 2003.

If that were not enough, most of these folks have been been saving and investing over four decades, when both stocks and bonds were appreciating more than 70 percent of the time. And except for the early 1990s and the 2008-2012 period, real estate prices have been rising as well.

But if current retirees could afford to spend more, why aren’t they? Some may be saving for what they suspect will be out-of-pocket medical expenses at some point. An analysis of their spending habits shows that health-care expenses do rise gradually while transportation and entertainment outlays tend to decline.

Still, current retirees could afford to spend a little more and “in some cases, a lot more,” according to the study. But many choose not to. Behavioral biases may prompt people to make sub-optimal decisions, the study suggests.

It’s also possible they have outside sources of income, like part-time work, consulting or ownership of rental properties that the study didn’t fully capture.

For those who have financial advisors, I have another theory. Many advisors tell clients to rely on their taxable accounts early in retirement while leaving their qualified plan assets alone so they can continue to enjoy the benefits of tax deferral.

Future Retirees May Not Be Lucky

Ackerley acknowledged the research leaves many questions unanswered and said BlackRock is conducting further research.

She also suspects that future retirees may not have it so easy for a variety of reasons. Chief among them is that only a sliver of Americans working today have a defined benefit plan. But that’s just the start.

Baby boomers are considered to be bigger spenders and skimpier savers than the previous “silent” generation that represents the bulk of today’s retirees. While boomers have participated in the bull market in stocks and bonds that began in 1982, most hadn’t accumulated the same amount of assets as prior generations when these markets took off.

People who had sufficient assets to retire before 2009 have earned double-digit returns on the equity portion of their portfolios early in their retirement years. Consequently, they managed to turn the sequence-of-returns risk to their advantage if they didn’t bail out of equities during the financial crisis.

Besides the prospect of lower returns going forward, other risks to future retirees includes stresses on Social Security and Medicare. While politicians have not shown any willingness to address entitlements, current benefit levels are unsustainable without dramatically higher taxes. What’s more likely are solutions that gradually chip away at benefits, combined with the expansion of stealth taxes on those same benefits.