Although socially responsible investing has grown steadily over the past 30 years — it is now at about $2.7 trillion, or 11 percent of all U.S. assets — it has been overwhelmingly individual money, invested via retail mutual funds. In the 401(k) world, SRI has been almost MIA.

Most consultants agree that only about 1 percent of defined contribution plans offer any sort of socially responsible fund as an option in their investment lineups, and those offerings are occurring largely at the noncorporate edges of the universe, in Taft-Hartley, educational, nonprofit and public sector plans. Narrowing the scope even further, often these funds aren’t a formal item on the investment menu, but are available only through open-ended brokerage windows.

So it was rather astounding to read, in a Mercer survey this year of 1,500 plan sponsors around the world, that 12 percent offer a socially responsible investment fund option. That number was down only slightly from an even more eye-catching poll three years ago, when 19 percent of the 129 U.S. plans responding answered yes — and 41 percent were seriously considering such a move. And these weren’t just the usual SRI-friendly organizations; fully 73 percent were corporate plans, evenly distributed by size.

To some degree, says Craig Metrick, the principal who heads Mercer’s U.S. responsible investing team, technical factors can explain the difference between his company’s surveys and other consulting firms’ experience. “Client interest comes up; they go away; they come back,” he explains.

More important, he points out, is that client interest is “growing pretty rapidly.” By his account, every week a different plan sponsor asks Mercer to add an SRI fund to its investment lineup — usually as a large-cap or large- and midcap domestic equity fund acting as an independent asset class.

“There are people who want to invest their money in things they believe in,” Metrick says. Although the financial crisis chilled a lot of the desire to experiment with new investment vehicles, he continues, “now that the markets have come back a little, they’re willing to open up some of those possibilities.” In addition, investor attention has been piqued as “more of these issues are cropping up in the news,” such as the BP oil spill in the Gulf of Mexico.

Add to those reasons one more: “The SRI industry is increasing and improving their research and their performance, putting themselves out in the media more,” notes Metrick.

Ah yes, performance. What about the cliché that investing according to any factors besides strictly financial ones will hurt returns? Well, there’s research backing every side of the argument, and the bottom line seems to be sector-driven more than social: Certain styles and sector biases flourish or flounder under certain macroeconomic conditions. SRI funds tend to shun Big Oil and favor technology, for instance, so they did great during the tech boom of the early 2000s and looked weak when oil peaked in summer 2008.

“Just like any other type of investment, there are going to be strategies that outperform and strategies that underperform,” Metrick says.

If plan sponsors don’t want to offer a fund option that’s bluntly labeled “ethical,” another approach is to try to nudge mainstream managers to take ethical issues into consideration when choosing the companies in their portfolios. To that end, Mercer has started rating all of the managers it recommends according to how well they integrate so-called ESG principles — environmental, social and governance issues — into their investment process, on a scale of 1 to 4.

Metrick concedes that few if any plan sponsors use the rating as a standard in selecting managers. Rather, he says hopefully, “it becomes something to talk to managers about, so that over time the plan’s average ESG ratings will grow.”

Fran Hawthorne is the author of the award-winning Pension Dumping: The Reasons, the Wreckage, the Stakes for Wall Street and Inside the FDA: The Business and Politics Behind the Drugs We Take and the Food We Eat. She writes regularly about finance, health care and business ethics.