“VIRTUE,” according to George Bernard Shaw, “is insufficient temptation.” But new research on the consumption patterns of the environmentally minded suggests that virtue and self-indulgence often go hand-in-hand.

A recent paper* by Uma Karmarkar of Harvard Business School and Bryan Bollinger of Duke Fuqua School of Business finds that shoppers who bring their own bags when they buy groceries like to reward themselves for it. For two years the authors tracked transactions at a supermarket in America. Perhaps unsurprisingly, shoppers who brought their own bags bought more green products than those who used the store’s bags. But the eco-shoppers were also more likely to buy sweets, ice cream and crisps.

Psychologists call this sort of behaviour “moral licensing”: the tendency to indulge yourself for doing something virtuous. Although this example may seem harmless (except to the shoppers’ waistlines), the results can be perverse. A study from 2011 on water-conservation in Massachusetts shows how. In the experiment, some 150 apartments were divided into two groups. Half received water-saving tips and weekly estimates of their usage; the other half served as a control.

The households that were urged to use less water did so: their consumption fell by an average of 6% compared with the control group. The hitch was that their electricity consumption rose by 5.6%. The moral licensing was so strong, in other words, that it more or less outweighed the original act of virtue.

Moral licensing does not seem to occur when virtuous conduct is obligatory. In one study, participants imagined themselves doing community service. Then they were asked to pick between two rewards: an indulgent one (a pair of designer jeans) and a practical one (a vacuum cleaner). If they were told to imagine that they had been sentenced to community service for a driving violation, they were much less likely to choose the jeans than if they pictured themselves as volunteers. The best way to get people to do good, it seems, is to make them feel bad about themselves.

* “BYOB: How Bringing your Own Shopping Bags Leads to Treating Yourself and the Environment”, Harvard Business School working paper, December 2014.