Let’s assume these efforts help and, over time, assaults and burglaries become rarer. What would the volunteer do next? One possibility is that they would relax and stop calling the police. After all, the serious crimes they used to worry about are a thing of the past.

But you may share the intuition my research group had – that many volunteers in this situation wouldn’t relax just because crime went down. Instead, they’d start calling things ‘suspicious’ that they would never have cared about when crime was high, like ‘jaywalking’ or loitering at night.

You might also like:

• How feeling bad changes the brain

• Should you trust your gut feelings?

• How anxiety warps your perception

You can probably think of many similar situations in which problems never seem to go away because people keep changing how they define them. This is sometimes called ‘concept creep’ or ‘moving the goalposts’ – and it can be a frustrating experience. How can you know if you’re making progress solving a problem, when you keep redefining what it means to solve it?

My colleagues and I wanted to understand when this kind of behaviour happens, why, and if it can be prevented.

Looking for trouble

To study how concepts change when they become less common, we brought volunteers into our laboratory and gave them a simple task – to look at a series of computer-generated faces and decide which ones seemed “threatening”. The faces had been carefully designed by researchers to range from very intimidating to very harmless.