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Individuals that have saved successfully for retirement evidently cannot kick the habit. Even after they have reached retirement age they continue to save, on average, 31% of income, new research shows.

In many cases this continued saving comes from income streams guaranteed for life, such as a traditional pension, certain annuities, or Social Security. So further saving may have little to do with financial security—and much to do with a routine that has served them well over the years. If you are looking for the top secret of affluent retirees, it may be just that simple.

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Retiree income flows from five primary sources, according to the research from fund company Vanguard. Guaranteed lifetime income is the biggest cut at 42%. Withdrawals from tax-advantaged accounts like IRAs and 401(k) plans are the second biggest source (20%), followed by pay from a part-time job (12%), withdrawals from savings accounts (7%) and from specialty accounts like a cash-value life insurance policy (4%).

The income source matters. Those who mainly get by on withdrawals from a 401(k) or other financial accounts reinvest about a third of what they take out due, say, to required minimum distribution rules. Those collecting guaranteed monthly income save only 25%.

This makes perfect sense. Lifetime income, by definition, never runs out. Those who get most of their income this way are under far less pressure to save anything at all. Meanwhile, those living off withdrawals from financial accounts, which can run dry, show a predictable concern with that possibility.

These are findings worthy of some study in government and pension circles. In coming years, a greater share of retirees will rely more heavily on their own savings, which could undermine spending in general and take a bite out of economic growth. On the other hand, those who get most of their income from withdrawals from financial accounts are more likely to work longer or part-time in retirement, which contributes to the economy and probably the individual health of those doing so.

The Vanguard study looked at households where the head was 60 to 79 years old, had at least $100,000 of investable assets, and at least one member of the household was fully or partially retired. This is an affluent, though not rich, group that continues to save and, in some ways may be doing so inappropriately.

Two-thirds of the money saved from income that comes from financial accounts goes into low-yielding savings vehicles. That might be by design—a desire to lower risk or save for a big purchase. But it might also be the result of inertia—required distributions left unattended. If such distributions are not needed for spending they might be better reinvested in growth or higher income accounts.

It’s tempting to assume that affluent retirees keep saving simply because they have the means to live as they wish and still have income left over. But that probably sells them short. They had to save or work hard for their pension to get there. It’s the habit that made it happen—and once established it’s tough to kick.

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