If you trade on the stock market, you may feel an inclination to invest in socially responsible companies, deemed supportive of the environment and active in combating global warming. But questions have arisen as to who does the deeming and how well it’s done.

The pressure is growing. Maine State Treasurer David G. Lemoine and Vermont State Treasurer Jeb Spaulding have joined 65 leading corporations and institutional investors in looking for companies that make climate change risk a top priority. And Vanity Fair magazine devotes most of its May issue to environmental activism. It stresses individual responsibility, warning that using too much toilet paper endangers the forests and buying a cell phone leads to the slaughter of gorillas in a region where metal for a component is mined. It calls “green tech” a bright new investment opportunity.

But a New York Times investigative financial reporter, Joe Nocera, covering a meeting of the huge financial firm TIAA-CREF, became interested in how come the Nike Corp. had been off and then on the firm’s list of socially responsible companies. It had been criticized for the treatment of workers in overseas factories but somehow had rejoined the ranks of “good companies.”

Mr. Nocera learned that TIAA-CREF had based its judgment on ratings by a small Boston company, KLD Research and Analytics, which operates a “social index” to achieve “greater corporate accountability and, ultimately a more just and sustainable world.” KLD evidently decided that Nike had cleaned up its act, but Mr. Nocera learned that the decision was based mostly on three-year-old news clips. The firm has only two dozen researchers who monitor 3,000 companies. They rely on media reports, blog interactions with activist organizations, and conversations with the company itself. The reporter found that to be little basis for judging a company to be “good” or “bad.”

Nike was using the same factories as Reebok and other shoe manufacturers including Timberline, but they stayed on the index while Nike was kicked off. Mr. Nocera asked why and was told that Nike had a special responsibility as the market leader.

Social-responsibility ratings generally can be haphazard and flawed. The article cites BP, the only oil company acceptable to the raters because of its early warnings about global warming. Yet in 2005 an explosion in a BP refinery in Texas killed 15 workers and injured 100 others. And it spilled oil in Alaska last year. Yet Exxon-Mobil remains on blacklists because of its skepticism about global warming, despite its good worker safety record and no oil spill since the Exxon Valdez went aground in 1989.

Mr. Nocera concluded that it would be nice if we could invest our money only in “good” companies, but “too bad it’s impossible.” The lesson seems to be to be wary of the ratings, make your own list, and judge for yourself.