Psychology of self-reward says bringing reusable bags to the grocery store doesn't guarantee a purely virtuous cart; research found shoppers also added more indulgent foods

You grab your canvas tote, feel good about skipping the plastic, and walk into the store ready to shop responsibly. But a 2015 study, ‘BYOB: How Bringing Your Own Shopping Bags Leads to Treating Yourself and the Environment,’ in the Journal of Marketing, found that the reusable bag might also be quietly nudging you towards the cookie aisle. Researchers found that bringing your own bags leads to more purchases of both eco-friendly and indulgent items in the same shopping trip.

The good deed that comes with a side of cookies
According to Harvard Business School's Working Knowledge publication, the researchers tested this by combining real shopping data with lab experiments. For the data part, they analyzed loyalty card data from one California grocery store location, following more than 936,000 purchases from nearly 6,000 households over time.

This design enabled them to compare what the same shopper bought on trips when bringing reusable bags to trips without them, helping to rule out the simpler explanation that 'bag bringers' are simply a different type of shopper altogether.

What the data actually showed
The study found that shoppers who brought reusable bags bought more organic groceries and more indulgent items, such as snacks and sweets, than they did on trips without them. The two categories didn't behave the same way, though. Indulgent items were often added to everything else, while organic items were often swapped in for regular, non-organic versions that were already in the cart. That difference matters a lot because it means that shoppers weren't just substituting healthier options for less healthy ones. They were expanding the cart altogether.

Why do we treat ourselves after being good
This isn’t the first time researchers have observed people rewarding themselves for doing something eco-friendly. A related study, ‘Do Green Products Make Us Better People?’ , published in Psychological Science by Nina Mazar and Chen-Bo Zhong, found that purchasing green products can build up a sort of moral credential in people’s minds, one that can end up loosening their self-control in unrelated areas afterwards. This general idea is often called moral licensing: doing something good can make us feel like we’ve earned some slack elsewhere. Reporting on the Duke research, Bollinger has compared this grocery bag version to the logic of telling yourself that you deserve dessert because you exercised that morning.

It's worth being careful with how far this goes. The 2015 paper describes a pattern observed at one grocery chain, with a series of controlled experiments to support it, not a universal rule about how all shoppers behave. The authors framed it as one piece of a broader, still-developing picture of how small environmental habits can ripple into other in-store choices.

When the effect disappears
Turns out, it really matters who chooses to use reusable bags. The extra indulgent purchases only showed up when shoppers chose, on their own, to bring their bags, according to the Duke University Fuqua School of Business summary of the research. But when participants were told a store required reusable bags, rather than leaving the choice up to them, the effect disappeared, and the additional indulgent buying went with it. In short, you seem to reward yourself only if you think you are personally responsible for the good deed. If the bag wasn't really your choice, there is less to be proud of, and therefore less to "earn" later on.

What this means for your next grocery run
None of this means that reusable bags are secretly bad, or that using one will automatically ruin your grocery budget or your diet. Most trips with a tote bag probably look completely ordinary. What the research does say is that the mental math we do around “being good” is more complex than it seems. A small, really good habit, like remembering your bags, can subtly open the door to a less healthy purchase, at least for some shoppers on some days.

So the next time you walk into the store with your tote slung over your shoulder, feeling a little proud of yourself, it might be worth a quick gut check before you hit the snack aisle: are you shopping for your values, or shopping for a reward?