How 'green bags' backfire: Eco-shoppers more likely to buy junk food, study finds

Shoppers, beware. In an ironic twist, it seems our attempts to remain environmentally conscious can have a not-so-desired effect on the way we shop.

A team of American marketing experts has confirmed that a trip to the supermarket often turns indulgent or expensive for those with reusable bags in tow.

A study released by Duke University's school of business shows that shoppers who bring their own reusable bags are more likely to buy organic produce over conventional products. But those same shoppers also tended to arrive at the checkout with indulgent junk food – a well-earned treat for "going green".

"In short, bringing your own bags changes the way you shop," write the authors.

The research, BYOB: How Bringing Your Own Shopping Bag Leads to Treating Yourself and the Environment by Bryan Bollinger of Duke University and Uma Karmarkar of Harvard Business School, was published in the Journal of Marketing.

Collecting loyalty cardholder data from a single California grocery chain store, the authors were able to compare the same shoppers on occasions when they brought their own bags with those when they arrived empty-handed.

According to the study, bringing your own bags encouraged some shoppers to make more "green" choices in the grocery aisle.

But for others, bringing the bag is enough.

"It is ... possible that by bringing the bag, shoppers would feel they had already taken measures to progress towards [green] goals."

Their reward? A packet of chips. Together with an organic blend of coconut oil.

Consumer psychologist and part-owner of Cummins & Partners media agency Adam Ferrier says the study is the first of its kind to draw such results.

"This study uncovers a finding that few would have predicted, yet ... makes sense," he says.

For authors Bollinger and Karmarkar, bringing your own bags is a form of "virtuous self-signalling" that allows some shoppers to indulge or act negatively afterwards.

As Ferrier explains: "Everyone has an internal ledger where we attempt to balance 'good' and 'bad' activities. The more good we do gives ourselves permission to treat ourselves to 'naughty' indulgences.

"It makes sense that if we are good and use our own recyclable bags, then we are allowed to be a little naughty and treat ourselves too."

In the US, only 29 per cent of customers used a BYO bag in their last shopping trip. This compares with Australia, where a 2012 survey from the Australian Bureau of Statistics showed nearly 60 per cent of shoppers always or often reuse bags when grocery shopping.

One project that encourages customers to return their plastic bags after use and buy reusable bags is the Coles REDcycle program.

"Since the program began, Coles has been able to divert more than 100 million pieces of flexible plastic from landfill across Australia," a Coles spokesperson says.

Despite this, Ferrier says the same behaviour would be found in Australia, but at a slowing rate.

"As the behaviour [of bringing your own bag] becomes the norm, then you will no longer see yourself as doing good as you're just doing the expected thing - and therefore the licence to treat yourself is lessened," he says.

As a new, plastic-free food movement takes over Australia and plastic bags could soon be history, it seems that consumers will continue to treat themselves - only when they feel deserving to do so.