When you ban plastic bags from supermarkets...

... people will take the supermarket baskets instead. As the New Zealand Herald reported a couple of weeks ago:

Since the move away from having plastic bags in their stores, some Countdown supermarkets have had the odd shopping basket go walkabout.

For the most part, Countdown says Kiwis remember to bring their own shopping bags and use them to carry their groceries.

However, one customer found out the hard way yesterday afternoon that some people clearly haven't got used to bringing their own bags.

When the customer walked into Countdown Lynfield in Auckland he could not find any of their distinctive green shopping baskets.

A staff member told him 175 baskets had been stolen.

Why steal a supermarket basket? Basket thieves could be rational, and weigh up the costs and benefits of their actions. The options for a would-be basket thief are to take the basket, or to buy a reusable bag, or to do neither and find some other way of transporting their groceries home. Assume the benefits of all three methods of transporting groceries are the same (after all, the outcome is that the groceries end up at home, either way). The difference is in the costs, and the rational basket thief will choose whichever option has the lowest cost.

Reusable bags have a monetary cost (at Countdown they start from $1 each). Not using a bag or a basket comes with a cost in the form of the awkwardness of transporting the groceries (balancing multiple items in your arms), or having them rolling around loose in the back of the car, etc. Taking a basket comes with a moral cost (the thief feels bad about taking the basket) and a social cost (other people will think badly of the basket thief if they find out).

How this calculus plays out will be different for different individuals. Those who feel moral and social costs greatly will buy the bags (or do without a bag or basket), while those who care less about moral concerns and how others think about them will take a basket.

How should the supermarkets respond? Increase the costs to would-be basket thieves. Make the security doors beep loudly when a basket is taken out, drawing attention to them. Have penalties for stolen baskets. Shame thieves by posting CCTV photos of them and their ill-gotten baskets. All of these options increase the costs of stealing a basket. They won't deter the most brazen basket thieves, but basket theft will reduce. When the costs of doing something increase, we tend to do less of it (including stealing supermarket baskets).