I check my phone. “Already inside, some good ales on.” I quickly flick my wallet open to see a £20 note, then open the door to my local pub. A wave of warm air, infused with familiar and somewhat questionable odours, washes over me. An evening of chatter and drinking with friends awaits. Unexpectedly, the pub has a promotion running, and I’m handed a voucher for a free drink as I walk in.
Here’s the question: over the course of the evening, will I now drink the same amount, or more, than I otherwise would have?
It’s a simple question. Let’s say I intended to get three pints of beer. Will I now get a free extra drink, and thus consume four pints, or substitute the free drink for one I’d buy?
I’m sure you can think of ways of answering this kind of question in a scientific way. Perhaps we could give a random selection of people vouchers and compare how many drinks they buy, on average, to people without vouchers. Understanding why they behave that way is much more complicated. Without asking why, though, we can’t begin to generalise and predict what might happen in other promotions, in other situations, with other resources.
Two recent studies have tried to answer a question like the one I posed, but at a much bigger scale. Candice Moy, in Geographical Research, examined whether owning a rainwater tank changes how people react to water becoming more scarce during a drought. More recently, Richard York, in Nature Climate Change, analysed whether development of renewable energy displaces fossil fuel energy, or simply adds to it. In other words, do rainwater tanks and renewable energy supplies function as extra resources, or substitute for resources in our existing systems?