Ethical shoppers will buy products with untested eco-friendly claims even when they are more expensive, says an Otago University study.
The results have prompted researchers to call for clearly defined national standards on ethical claims - preferably monitored by an independent agency - to stop companies "greenwashing" their products.
The marketing study found many people with strong ethics about consumption told interviewers they were sceptical about unverifiable claims.
However when a second, larger group was asked to make a snap decision on laundry powder, shoppers with ethical leanings were more likely than others to buy a packet labelled "eco-friendly", "natural ingredients" or "no animal testing".
Thirty-eight per cent rated the eco-friendly label ahead of a 12 per cent price cut and the natural ingredients claim, which they ranked equally.
The other 62 per cent were also more likely to buy the "eco-friendly" laundry powder but by smaller margins. They were more interested in the price cut, which was twice as important as any other factor.
The study found ethical claims influenced busy shoppers' decisions more than many cared to admit.
"If these claims are no more than puffery, consumers will be misled and deceived, and policy makers should examine whether standards-based claims, monitored by an external independent body, would facilitate more even competition and promote greater consumer protection.
"Such a stance would not only benefit consumers but enable those manufacturers who have created a genuine competitive difference to promote this more effectively."
Lead author Professor Janet Hoek said even self-described ethical con-sumers didn't have time to assess detailed evidence on every product.
"We urgently need some clear standards that are easy for people to understand and that they can trust.
"For consumers, who spend literally a couple of seconds making food choices, there's nothing to identify which of these are trustworthy labels and which have a stronger element of self-interest in them."
Price was the decider for most but a significant minority ranked ethical considerations at least as highly, enough for manufacturers to make claims in their marketing strategy.
Ecostore founder Malcolm Rands, whose company makes skincare products, laundry detergents and cleaning supplies, said greenwashing was a major problem, especially for products like laundry powder where consumers could not be sure what chemicals were used.
He thought it should be illegal for firms to make ethical claims which they could not back up but predicted this would be complicated, as his company had to use several different certification agencies to prove its environmentally friendly claims.
The Otago University findings are similar to an American study last year which found sales of a United States supermarket's two most popular bulk coffees rose by 10 per cent when they were relabelled as Fairtrade.
However that changed when the price was raised 8 to 9 per cent. Sales held steady for the higher-priced coffee but dropped 30 per cent for the lower-priced one when the price was raised.
New Zealand and Australian food ministers last month ruled out "traffic light" food labelling - giving a green, orange or red light for the level of fat, sugar and salt - but said they would come up with an alternative this year.