You're in the supermarket. Imported beef mince, shrink-wrapped vegetables and cleaning spray are already in your basket. Then you toss in some organic apples and feel a flicker of moral relief. Surely that small green gesture lightens the load?
Authors
- John Everett Marsh
Reader in Cognitive Psychology, University of Lancashire
- Patrik Sörqvist
Professor of Psychology, University of Gävle
Not quite. Objectively, every extra product increases your carbon footprint. But psychology research reveals a curious illusion : when we add eco-friendly items, we often judge our shopping basket as having less impact on our carbon footprint than before.
This mental glitch is called the negative footprint illusion , and it matters for how we shop, how businesses market themselves and how governments design climate policies.
The illusion has been demonstrated across dozens of studies. In a typical experiment , people are asked to estimate the carbon footprint of 150 standard houses. Then they estimate the footprint of those same houses plus 50 eco-houses. Mathematically, the second total must be higher - there are simply more houses. Yet participants often judge the mixed set as lower.
In other words, adding a "good" item doesn't just seem to cancel out a "bad" one. It creates a false impression that the total footprint has gone down, when in reality it has gone up. And the more "green" items you add, the stronger the illusion becomes.
What's striking is how stubborn this bias is. It occurs among people with strong environmental values , people with scientific training and even among experts in energy systems. Education and numeracy don't protect us. This isn't a problem of knowledge, but of how the mind simplifies complex judgements.
Why does it happen?
The main culprit is averaging . Instead of adding up the total impact, we unconsciously average the mix. Toss in a few low-impact items and the "average impression" improves, even though the overall footprint goes up.
Our memory also plays tricks. If a sequence ends with an eco-friendly item, that last impression weighs heavily and colours the whole set . Likewise, when items are arranged irregularly, we find it harder to keep track of how many there are, so we default to averages rather than totals.
Psychologists have long shown that even when people are told about a bias, they often fall right back into it. Our latest experiments suggest the same applies to the so-called negative footprint illusion. That suggests it isn't just sloppy reasoning but a deeper mental tendency: the mind simplifies.
The illusion may seem harmless in a lab, but it has real-world consequences when it comes to shopping, for example.
Businesses have also learned, consciously or not, to exploit this bias. A fast-food chain might showcase paper straws while still promoting beef-heavy menus. A hotel might advertise its towel-reuse policy while quietly expanding its energy-hungry facilities. These green cues create a halo that spills over to the whole brand.
Even well-intentioned policy nudges can misfire. Offering more green-labelled choices is often assumed to drive better behaviour. But if those choices mask the real cost of consumption, they may backfire - encouraging people to consume more under the false impression of virtue.
Can it be fixed?
The good news is the illusion can be reduced. One promising approach is "summative priming" : nudging people to think in totals rather than averages. In experiments, participants who first completed simple "totalling" tasks were later more accurate in judging carbon footprints.
Research shows that when eco-friendly items appear at the end of a list, they distort overall impressions more strongly. Placing them earlier makes the illusion weaker. Likewise, when items are arranged in a regular, predictable structure, people find it easier to keep track of totals and are less prone to averaging errors.
These tweaks won't eliminate cognitive bias entirely, but they show that design matters. Product labels, online platforms and policy communications can all be shaped to help people think in terms of totals rather than averages.
Climate change is driven by millions of everyday decisions: what we buy, what we eat, what we throw away. Understanding the psychological biases behind those decisions is essential.
The negative footprint illusion reminds us that even well-intentioned, environmentally conscious people can misjudge the true impact of their actions. Simply offering more green options isn't enough. If those options distort our perceptions, they may slow genuine progress.
The challenge, then, is not only to provide information - carbon scores, eco-labels, green badges - but to present it in ways that match how people actually think. That means designing interventions that highlight totals, not averages, and that help consumers see the cumulative impact of their choices.
Climate change is a global problem, but it is fuelled by small misjudgments at the individual level. By recognising how our minds work, we can design smarter tools, better policies and more honest messages - and nudge ourselves towards the sustainable future we urgently need.
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John Everett Marsh is a Reader in Cognitive Psychology at the University of Lancashire in the UK. He is also a Visiting Associate Professor at the Luleå University of Technology in Sweden and Bond University in Australia. He receives funding from Riksbankens Jubileumsfond.
Patrik Sörqvist receives funding from Stiftelsen Riksbankens Jubileumsfond and The Swedish Energy Agency.
