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?

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

© The Conversation