BOSTON (MarketWatch) -- What's the best way to motivate Americans to save, invest and prepare for retirement? Some behavioral finance experts suggest using the carrot. Others suggest using the stick. And still others suggest using a combination of carrot and stick.

As for me, I suggest the use of statistics. Consider just a sampling of the numbers that have been released this year:

There's $4.23 trillion in individual retirements accounts, but that figure hides the fact that very few Americans contribute to an IRA and even when they do the amount is small.

On average, just 10% of eligible Americans contributed to an IRA for the years 2000 to 2002, according to the latest issue of EBRI Notes. And in 2004, the median contribution to a traditional IRA was just $2,300, according to the Investment Company Institute. The maximum you could contribute to an IRA in 2004, by the way, was $3,000 or $3,500 for those 50 and older.

Now you might say that's not so bleak given that working Americans are presumably saving for retirement using an employer-sponsored plan, such as a 401(k), 403(b), 457 or Thrift Savings Plan. But again, the numbers are somewhat depressing.

There are nearly 100 million Americans age 21 to 64 working full-time, full-year. But of that number, just 60% or 58.4 million work for an employer that sponsors a retirement plan, and only 52.7%, or 50.8 million participate in a retirement plan.

That means roughly half of all working Americans don't participate in a retirement plan or don't have an employer-sponsored plan in which to participate. It also means that a huge number of adult Americans -- by my estimate 150 million of a potential 200 million -- aren't saving for retirement in any meaningful way, if at all.

Retirement risks

According to the Society of Actuaries' 2007 Risks and Process Retirement Survey, roughly half to 60% of retirees worry about three things: the cost of health care, the effect of inflation on their nest eggs and not being able to maintain a reasonable standard of living for the rest of their life.

Those worries are justified given the lack of savings in America. But what's really bothersome is the degree to which those who aren't worried should be.

Consider, for instance, health-care costs. Fidelity Investments estimated earlier this year that a 65-year-old couple retiring today would need $215,000 set aside just to pay for medical expenses over a 20-year span. And if that wasn't depressing enough, other estimates are even higher.

Paul Fronstin of the Employee Benefit Research Institute, for instance, said a 65-year-old couple retiring today would need, assuming average life expectancy of 82 for men and 85 for women, more than $300,000 set aside to pay for health-care costs (premiums and out-of-pocket expenses) in retirement, and more than $550,000 if the couple lives to age 92.

What's even more depressing is that neither the EBRI nor Fidelity estimates factor in the cost of nursing homes, long-term care or assisted-living facilities, or home health aides. And those costs are staggering.

According to the MetLife Mature Market Institute, it costs $69,000 per year for a semiprivate nursing-home room, $35,628 per year for a unit in an assisted-living facility, $19 an hour for a home health aide and $61 per day for an adult day care center. Where's that money going to come from?

Retirement expenses

Retirees and would-be retirees are also right to fret about maintaining their standard of living. Consider, for instance, these numbers: The median household income (half above, half below) in America is $48,451 and the average is $65,527, according to the U.S. Census Bureau. But in retirement, income falls dramatically.

The average total income for those 65 and older in America is $25,610, and the median was a meager $16,770, according to EBRI Notes. That means retirees are living on roughly one-third of their preretirement income. And that's a far cry from the 70% to 80% income replacement experts suggest Americans need to maintain their preretirement standard of living.

Besides not having the income to maintain a similar standard of living, retirees will face expenses that are certain to rise faster than the average rate of inflation.

Consider, for instance, the results of the 2002 Consumer Expenditure Survey. On average, retirees spent 32.6% on housing, 14% on food, and 13% on health care. But that's the average. What's interesting is the degree to which money spent on health care in retirement changes over time.

For instance, those 55 to 64 spend 6.8% on health care, those 65 to 74 spend 11.2% and those 75 and older spend 15.1%. That percentage rises in part because the cost of health care is rising twice as fast as the core rate of inflation (less energy and food), 5% vs. 2.3%, according to the U.S. Bureau of Labor Statistics. But it also rises because older retirees tend to spend more on health care than younger retirees.

Source of retirement income

So where do retirees get their income once in retirement? Again, the numbers are depressing (and deceiving). On average, retirees get 39.8% from Social Security, 23.7% from earnings, 19.4% from pensions and annuities, 15.4% from assets (IRAs and the like) and 1.9% from other sources, according to EBRI Notes.

But the composition of the income changes dramatically based on income. Retirees in the bottom fifth of income, those with less than $8,261 in 2006, got 87.6% of their income from Social Security while those in the top fifth of income, those with greater than $34,570, got 36.4% from earnings, 22.6% from pensions, 20.5% from assets, and just 18.5% from Social Security.

The moral of story

If you are among the 150 million who are not saving for retirement, now would be a good time to do so. If you are among the 50 million who are saving for retirement, now would be a good time to save more.

If you are among those who aren't worried about health-care costs, inflation or maintaining a standard of living in retirement, now would be a good time to start worrying.

If you are among those who worry about retirement risks, now would be a good time to do something about it: Set aside money for health care, for instance.

And if you are among those who don't know what your sources and composition of retirement income will be, now would be a good time to figure that out. After all, waiting to see how things might work out isn't the world's best plan.