Look at the humble sum in your 401(k) or a 403(b) retirement savings plan at work.

Maybe you think it would look a little more encouraging if you could just get a decent raise and save more.

To be sure, more savings would help most people. But your failure to stash more cash for the future probably isn't all that's ailing your 401(k).

A recent study of 1 million 401(k) accounts by pension investment advisers Financial Engines shows that Americans are betraying their futures in two ways: Many aren't saving enough, but the majority also is doing such a poor job of investing that they aren't giving their hard-earned savings a chance to bulk up.

In fact, with a little more investing savvy, the people in the study would be on course to have about 28 percent more wealth at retirement than they are now likely to have, if they have 20 years of investing to go.

Those with incomes of $25,000 or less are in the worst shape — suffering from a nasty combination of tiny savings and investing errors. But more income doesn't necessarily solve the problem: In the highest-income group — people earning $100,000 or more — only 37 percent are investing their money effectively, according to the research.

Among all income groups, people were committing two major mistakes with risk. Many were taking on so little risk with their investments that they won't accumulate the money they will need to pay for basic retirement living expenses. Others are taking on needless risk that is likely to undermine their savings at some point — even if their investments look healthy now.

A major mistake is investing too much money in the stock of the company that employs the account holder. About 40 percent of people are relying too much on a single stock, a dangerous proposition because even the stocks of solid companies can turn into disappointing investments for years in a bad cycle for the economy or an industry. And no one can predict when the downturn might come.

Financial Engines found that people with the lowest salaries and those over 60 were the most overexposed to their company stock. One in 4 people over age 60 had 25 percent or more of their retirement savings in their company's stock. Experts say that less than 10 percent is advisable.

The biggest mistake by young people is passing up free money their employers will give them if they contribute to their 401(k). Almost half of workers in their 20s lose matching money because they don't contribute enough.

Financial Engines found people in all income groups saying they weren't willing to contribute enough to obtain all the available free money. In the $50,000 to $100,000 group, 17 percent were saving so little they were giving up money.

Besides passing up matching money and buying too much company stock, large numbers of workers undermined their investments by choosing every mutual fund offered, or by putting half into the most conservative investment offered, such as a money market fund, and the other half into something risky, like a small-cap growth fund.

When Financial Engines removes inflation and fees, investments grow about 4.6 percent a year when invested well, and 3.3 percent when making mistakes.

Financial Engines' optimal portfolio for a 40-year-old would be about: 15 percent in bond funds, 20 percent in a large-cap stock fund, 20 percent in a mid-cap stock fund, 7 percent in a small-cap stock fund, 33 percent in an international stock fund and 5 percent in company stock.

For a 60-year-old, it would be 41 percent bonds, 5 percent stock, 26 percent international fund, 16 percent large-cap fund and 12 percent mid-cap fund.