Here’s a story that caused a bit of buzz in the office earlier in the week. Apparently, researchers in Toronto found that university students who bought “green” products in controlled tests were less generous with strangers, and more likely to tell a fib. The researchers suggest that this is an example of “licensing”: when people act virtuously in one domain, they feel entitled to shirk in another.

Or, to put it in holiday terms: sometimes, when we’re extra nice, we feel like it’s OK to be a little bit naughty. (Santa won’t mind, right?)

To me, this research hints at the limits of green consumerism.  I’m certainly not suggesting that there’s something wrong with being “nice”—buying organic, say, or recycling. Those are good things! Yet I’m Scrooge-ishly skeptical that simply offering people a handful of “virtuous” options, among a sea of less-virtuous ones, will do much to advance the cause of sustainability.

Licensing isn’t limited to choices that we actually make: simply being offered a “virtuous” choice can make us feel like we’re entitled to be naughty. Consider the study showing that diners presented with healthy options on a fast food menu are more likely to order french fries. Knowing that a salad was available made people feel, subconsciously, like healthy eaters; so they felt entitled to splurge on deep-fried potatoes. (People are weird, no?)

Once you start looking, you can see the “virtuous purchases” frame—the idea that better purchases will make you a better person—all over the place. The tactic is obvious enough in advertising.  But it even affects our political discourse.

Take, for example, climate scientist Jim Hansen’s objection to a cap and trade climate policy, on the grounds (among others) that it makes personal virtue irrelevant. With cap and trade, he complains, buying a Prius simply lets someone else buy a Hummer, so people will have no way to exercise—or take credit for—their virtue.

Hansen’s being obtuse here: the obvious policy solution is to let people buy and retire pollution credits, just as schoolkids did in the acid rain cap-and-trade program. But beyond the basic factual error, he’s focusing on a triviality. Giving people the opportunity to feel more virtuous than their neighbors is simply irrelevant in good policymaking. In fact, a reliance on personal virtue may ultimately be self-defeating; the Prius buyer could always reward her virtuous self with a trip to Hawaii, while doing nothing to remove a single gas-guzzler from the roads.

So instead of hoping for virtuous people—and designing policies that help individuals and consumers feel like they’re making a difference—we should be working towards virtuous policies. The best climate policy of all would take personal virtue entirely out of the equation. It would act invisibly, altering the DNA of the economy, changing the range of choices that are on offer in a way that makes cheating, in the aggregate, simply impossible. That’s why, when forced to choose, I side with cap and trade over carbon taxes. A well-designed cap-and-trade policy offers greater certainty in its results; there’s no cheating allowed, and no virtue required.

So if we stop focusing on making people virtuous, and start focusing on making our policies virtuous, we actually put our goals within reach—and we won’t have to rely on the hope that, Grinch-like, our hearts might somehow grow large enough to overcome our flawed natures.