Investing isn't easy. Even Warren Buffett counsels that most investors should invest in a low-cost index like the S&P 500. That way, "you'll be buying into a wonderful industry, which in effect is all of American industry," he says.

But there are, of course, companies whose long-term fortunes differ substantially from the index. In this series, we look at how individual stocks have performed against the broad S&P 500.

Step on up, PNM Resources (NYSE:PNM).

PNM Resources shares have deeply underperformed the S&P 500 over the last quarter-century:

Pnm Sp

Source: S&P Capital IQ.

Since 1987, shares have returned an average of 2.6% a year, compared with 9.7% a year for the S&P (both include dividends). One thousand dollars invested in the S&P in 1987 would be worth $19,200 today. In PNM Resources, it'd be worth $2,300.

Dividends accounted for a lot of those gains. Compounded since 1987, dividends have made up 78% all of PNM Resources' returns. For the S&P, dividends account for 39% of total returns.

Now have a look at how PNM Resources earnings compare with S&P 500 earnings:

Pnm Earnings

Source: S&P Capital IQ.

Underperformance here, too. Since 1995, PNM Resources' earnings per share have declined, compared with 6% a year growth for the broader index.

What's that meant for valuations? PNM Resources has traded for an average of 18 times earnings since 1987 -- below the 24 times earnings for the broader S&P 500.

Through it all, shares have been disappointments over the last quarter-century.

Of course, the important question is whether that will continue. That's where you come in. Our CAPS community currently ranks PNM Resources with a four-star rating (out of five). Care to disagree? Leave your thoughts in the comment section below, or add PNM Resources to My Watchlist.

Fool contributor Morgan Housel has no positions in the stocks mentioned above. The Motley Fool has no positions in the stocks mentioned above. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.