By Mary Kay Kleist

(CBS) — For a lot of shoppers, it’s the right thing to do: Bring reusable bags to the grocery store.

But as CBS 2’s Mary Kay Kleist reports, something interesting happens when shoppers use those bags and it has nothing to do with being green.

It’s become a common site in stores: responsible shoppers doing their part to help the environment with reusable bags.

But does using those bags change what they buy?

Harvard researchers wanted to find out.  They analyzed shopping patterns from loyalty card users and then compared shoppers who brought their own bags to shoppers who didn’t.

“On the trips where people brought their bags, they seemed more likely to buy organics,”  researcher Uma Karmarkar says.

But here’s the part that surprised her: While bag users bought more things like organic fruits and vegetables, they also bought more “indulgence” items.

“They were also more likely to put an indulgence in their basket. We define indulgence as cookies, ice cream, and potato chips,” Karmarkar says.

So, what makes people who are trying to be environmentally friendly more likely to treat themselves?

“When you do something good in one domain, you potentially, unconsciously are more likely to indulge in another one,” she says.

And, what’s wrong with a little pat on the back?

“I’m helping a little bit, helping save the environment, so I feel good. I guess that that’s a reward,” shopper Julie Irvine says.

One more interesting note: The findings only hold true for bag users who don’t have kids and only when the price is right.

Mary Kay Kleist