For years, I've preached that corporate social responsibility helps the bottom line. Respect the environment, your employees, and the community, I argued, and they'll not only respect you back; they'll buy your products. Unfortunately, I've never been able to prove it or even find a study to back me up. In fact, most research shows that consumers aren't willing to pay more for a socially responsible product. They want the best deal, period.

And the truth is, that's the way it should be. Companies aren't moral beings. They exist to make money for their shareholders by hanging on to customers. When a firm's top brass go on a social-responsibility offensive, you can be sure it's self-interested: They want to increase their profits by burnishing their public image, cutting costs, or avoiding even more costly regulation.

I've been thinking about this more often lately as a raft of new companies rush to embrace social responsibility. PepsiCo chairman Indra Nooyi recently announced plans to take a Frito-Lay factory off the power grid and run it almost entirely on renewable fuels. Texas Instruments has a new, green semiconductor plant. Even Wal-Mart, long reviled for its paltry pay and pathetic benefits, is getting in on the act. This past fall, C.E.O. Lee Scott unveiled a broad employee health-care package with premiums as low as $5 a month; before that, he revealed plans to install low-energy lighting in Wal-Mart's stores and switch the packaging for its fresh produce to plastics made from environmentally friendly corn sugars.

Like all good top managers, these executives are doing what they're supposed to do according to the current rules of the game. Those low-energy lightbulbs will not only reduce Wal-Mart's carbon dioxide emissions by 35 million pounds a year; they will also save the company (by its own estimation) $2.6 million annually. That new, green packaging turns out to be cheaper than the old. Adopting better health coverage is not just an effort to blunt public criticism about stingy benefits (the same stingy benefits that contribute to the great deals we get as consumers) but is perhaps also a bid to preempt action by several states that have recently threatened to require big employers to provide workers with even more generous health benefits. (See other cost-effective changes made by companies.)