TODAY's consumers want to spend their cash on responsible companies' products, Paul Polman, Unilever's CEO, told me recently. "They are increasingly willing to vote with their wallet."

You hear the same from many corporate leaders. Consumers are insisting that the products they buy do not destroy the planet or come from sweatshop factories. Yet I detected some frustration alongside Polman's assertion. He said some of those same consumers were not getting the ethical message. Unilever has pledged that all its palm oil will come from certifiably sustainable sources by 2020. The group is talking to the Indonesian government about investing more than ?100m in a processing plant in Sumatra so it can trace its palm oil purchases more accurately. Responsible consumers should be pleased with what Unilever is doing. But they don't seem to care.

"We've seen limited consumer understanding on buying products that come from illegal deforestation," Polman said. " So you have to do an educational programme."

It is an old story. Consumers tell market researchers they care about products being responsibly produced. But when they get to the supermarket, they buy the same goods they always did. It is called the 3:30 paradox: 30% of consumers call themselves ethical shoppers but only 3% are. There are signs of this changing. Worldwide sales of Fairtrade goods, which guarantee farmers a minimum price, rose 27% to ?4,4bn in 2010. There were some enormous increases in individual markets. In Australia and New Zealand, Fairtrade purchases were up 258%. In South Africa, they increased 315%. In the UK, they rose 5%. Yet the overall totals are small. Whatever the market research says, concerned shoppers are a tiny minority.

In Why Ethical Consumers Don't Walk Their Talk, a 2010 study in the Journal of Business Ethics, three Australian-based academics said there were two possible explanations for this "intentions-behaviour gap".

The first was consumers gave market researchers what they believed were socially acceptable answers before buying what they wanted.

The second, which the writers preferred, was that consumers really did intend to be ethical but, when they got to the shops, it proved too difficult. They could not find the goods, or they were too expensive, or the kids distracted them.

So why are companies bothered if consumers aren't? First, some executives, such as Polman and Mark Parker, head of Nike, truly worry about the sustainability of the resources required for their products.

Second, they want to get the campaigning organisations, such as Greenpeace, off their backs.

Third, even if not all consumers are as punctilious in their purchases as they say they are, there are still small, often affluent, groups that care.

How can companies encourage more shoppers to buy their ethical goods? The Australian writers suggest well-tried tricks: make them visible and do some price-cutting.

© 2012 The Financial Times Limited