WASHINGTON — How diversified are your retirement savings?

I knew I was perhaps too concentrated in Washington Post stock (about 17 percent) and maybe a little light when it came to bonds (4 percent), but I figured I was pretty well off when it came to the rest of my stock investments. After all, I had investments in four stock mutual funds.

That was before I looked closer. Two stock mutual funds accounted for most of my holdings, and it turned out they had a lot in common. Two of their top five holdings were identical: Citigroup and Bank of America made up more than 4 percent of the holdings of one mutual fund and more than 8 percent of the holdings of another.

Maybe not as diversified as I thought.

And it turns out I'm not alone. Most of us are not as diversified as we should be when it comes to retirement investments. At its simplest, diversification means investing in different types of assets.

When it comes to retirement savings, the choices are basically stocks, bonds and cash and cash equivalents, such as savings accounts, certificates of deposit and money-market funds. Put all your money in a single stock, and you may be exposed to more risk than you realize — as employees at Enron and WorldCom learned, to their sorrow. At the other extreme, invest completely in safer but lower-yielding investments, and you may be earning less than with a more diversified portfolio.

But you also need to spread your risks within each asset category. That's why so many investors choose mutual funds, which allow them to own a small portion of a large number of investments. But buying more than one mutual fund may not add much to the diversity of your investments if the funds are investing in the same companies. And sometimes they are, as I found.

Edwin J. Elton and Martin J. Gruber, Nomura professors of finance at the Stern School of Business at New York University, have spent a lot of time looking at why this is so. One reason, according to Elton, is that there isn't much diversity among the choices employees are offered in their 401(k) plans. Many companies rely on mutual funds from a single family of funds, and they often offer more than one fund with similar stock-picking outlooks — for instance, choosing companies with strong earnings and revenue growth. In some cases, the overlap in stock holdings within a fund family is as much as 34 percent of total net assets, they found.

One reason is that different fund managers within the same family (Fidelity, Vanguard, T. Rowe Price, etc.) are often looking at the same economic forecasts and analyst research when they're choosing stocks.

The other reason investors may be less than ideally diversified is their own behavior, Elton said. "They tend to chase performance. They tend to overinvest in their own company's stock. They tend not to rebalance." Rebalancing is adjusting your holdings to keep the mix of investments you choose — say 75 percent stocks and 25 percent bonds — if a soaring stock market shifts that balance.