NEW YORK (CNN/Money) - Grantham Mayo Van Otterloo is one of the best-performing fund families in the country. With some of the world's ritziest clients, including the Yale and Princeton endowment funds, it has status to burn.

But odds are you've never heard of it, because for most of its 26 years the Boston-based GMO has catered to those with millions to invest, not mere thousands.

In recent years, however, the firm has begun subadvising a handful of retail mutual funds, providing small investors access to one of the most successful -- and one of the quirkiest -- shops in money management.

How successful?

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Over the past five years, GMO's 34 institutional funds have beaten, on average, nearly 80 percent of all funds in their categories, with more than a third landing in their categories' top 10 percent.

True, GMO has the advantage of low, institutional-level expenses. Nevertheless, there's serious investment talent here and some seriously unusual portfolios.

At GMO "we view value as almost morally the right way to invest," says Chris Darnell, the firm's director of quantitative research. All of the equity funds, including those focused on growth, have at least one value component, which comes in a couple of flavors.

For example, the portfolio of Vanguard U.S. Value, which is subadvised by GMO, includes "traditional" value stocks that just plain look cheap, such as Bank of America, which has a price/earnings ratio of 13, based on the previous 12 months' earnings. It also has "intrinsic" value plays such as Pfizer (P/E: 20).

5 ways to get GMO It has some of the world's ritziest clients, but it also provides management for these five funds. Fund  Ticker  YTD Return  Calvert World Values International Equity  CWVGX  10.7%  Evergreen Asset Allocation  EAAFX  11.6%  Evergreen Large Cap Value  EILAX  12.7%  USAA Income Stock  USISX  9.1%  Vanguard U.S. Value  VUVLX  14%   Source:  Morningstar, Inc.

GMO's definition of intrinsic value involves companies whose often higher stock prices are offset by pristine balance sheets and histories of impressive returns on investment -- companies with a high level of quality that has not yet been fully recognized by the market.

The value component isn't terribly shocking: Throwing a little of it into a growth portfolio is not unlike sneaking spinach into your kids' mac 'n' cheese.

More surprising is that all of GMO's funds, including those with a value orientation, invest some money using a momentum strategy, basically buying stocks whose prices are on the rise.

This is often considered a high-risk game, but GMO uses momentum to control risk. While it believes a value bias will pay off in the long run, that slug of momentum, which typically represents 30 percent of a fund's assets, helps support returns when the managers' beloved value strategy is out of favor.

To profit from this kind of diversification, however, says Arjun Divecha, who oversees the firm's emerging markets portfolios, "You need to keep value and momentum in separate cages. If you let them mate -- as with investors who bought the lowest-priced momentum stocks in 1999 -- they produce depressingly tame offspring, and you get the benefits of neither strategy."

To keep the beasts apart, GMO splits each fund into subportfolios following different strategies. To bring it all together -- traditional and intrinsic value, price momentum, earnings momentum and the asset-allocation stats the firm constantly compiles -- GMO relies heavily on quantitative research.

"Emotions are an investor's greatest enemy," says Divecha, and the quant work keeps the firm as close as possible to its goal of "relentless rationality."

Individual investors can access this dispassionate approach through five funds. GMO's been steering no-load Vanguard U.S. Value to category-beating results for three years. And in July, GMO began subadvising no-load USAA Income Stock. The load funds it subadvises, with more mixed results, are Calvert World Values International Equity and the Evergreen Asset Allocation and Large Cap Value funds.

In the long run, GMO's rationality has done nicely by investors, but it hasn't always benefited the firm. In the late 1990s, clients didn't want to hear about a bubble.

But Jeremy Grantham, the firm's co-founder, just couldn't keep quiet: "It was kind of a preacher instinct," he says, "sort of, 'Abandon hope, ye miserable, creeping people.'"

Instead, clients abandoned GMO, taking more than a third of its assets with them. The clients came back, but with the recent rally, GMO is again thundering from the pulpit.

U.S. stocks are substantially overvalued, it says; inflation-protected Treasuries and foreign small-caps look better.

Braced for the inevitable outcry, Tony Ryan, head of client services, just smiles. "We can't tell you when the wave will hit the beach," he says. "But we are very sure of our ability to predict the tide."