Have you ever felt like things are getting worse? One study, recently published in Science magazine suggests that this may be because things are actually getting better.

Daniel Gilbert, author of Stumbling on Happiness, and a team of psychologists have found evidence for what they call "prevalence-induced concept change". What this means is that how we apply concepts changes depending on how many examples of that concept are around. In one experiment, psychologists showed subjects coloured dots on the spectrum of blue and purple. When experimenters showed fewer blue dots, subjects began to identify more of the dots from the middle of the spectrum as blue. In other words, when fewer really blue dots were there, even purple started to look blue. As prevalence decreases, the concept of blue expands.

The study found the same pattern applies to more complex phenomena than colour, such as the emotional expression of human faces and abstract ethical concepts. Consider how this might work with the concept of laziness. In army boot camp, where everyone is constantly at attention, there are relatively few examples of obvious laziness. This can make the concept of laziness expand to include behaviour that might otherwise seem normal; slouching, imperfectly made beds, scuffed boots, may all come to be seen as examples of laziness.

Gilbert et al. suggest that prevalence-induced concept change may cause unjustified pessimism, because as the prevalence of bad things decreases more things start to seem bad. The authors warn that this can be dangerous for people and organizations that set out to solve problems. Suppose I decide to improve my diet by eliminating junk food. Once I've cut out the candy and deep-fried foods, bread and ice cream may fall under suspicion. If I continue to clean up my diet, red meat and sugary fruits may be the next to go. Yet the concept of unhealthy food expands until every morsel seems potentially toxic.

On a larger scale, the authors argue that "although modern societies have made extraordinary progress in solving a wide range of social problems...the majority of people believe that the world is getting worse." Why? This might be in part due to the effect of prevalence on our concepts. Even when things get better, people will tend to perceive them as getting worse. Expanding concepts of problems mean that we are unlikely to appreciate the progress we have made towards solving them.

On the one hand, this can be a good thing. The ratcheting effect means that we can notice more and more subtle versions of the bad things we want to eliminate. In the above example, I will have an objectively healthier diet the more closely I scrutinize my food. Concept-change can motivate continued progress. However, there is also a down side. As we all know, strict diets involve sacrifices, inconveniences, and effort. At some point the costs of maintaining such a diet exceeds the benefits. Is it worth giving up ever enjoying a restaurant dinner with our family to shave off one more percent of body fat? Is the stress and pessimism that comes along with seeing even the most minor things as problems really worth it if the problems are not, objectively speaking, very serious?

According to the study, the effect also works in reverse: when prevalence rises, concepts contract to exclude ambiguous instances. When subjects were shown more blue dots, they only identified the really blue ones as blue. That means that when bad things become very common, they just won't register as very bad anymore. They start to seem normal, and we only notice the most extreme examples. Although the authors do not explicitly draw the inference, their results seem to support a"boiling frog" effect. According to fable, frogs that are put into boiling water will jump out immediately. But if they are heated slowly, they will cook to death before they notice the danger. The same metaphor has been used to describe the response to climate change or people who remain in abusive relationships. Contracting concepts may contribute to this effect.

Prevalence-induced concept change, then, poses two dangers. One is that it can make us worry too much about things that are not really that bad. It makes us pessimists in the face of improvement. The second is that it may help normalize things that ought to worry us, masking worsening problems.

How should we deal with this? One way is to link our concepts to objective measures, and to check in with those measures to help us notice if our concepts are changing. If we want to improve our diet, we should decide on our overall goals in specific terms and which foods we want to eliminate. Before expanding the don't-eat list, we should decide whether doing so serves our goals. How healthy do we want to be? Similarly, if we want to take a reasonable attitude towards something like climate change or worsening relationships, we ought to check in with objective and measurable indicators regularly to ensure that we are not walking blithely towards disaster.

However, there is nothing inherently wrong with changing concepts. Sometimes it makes sense to expand or contract our concepts as things change. When we've tackled one problem, there's no particular reason why we shouldn't move on to the next worse once. It is, however, usually worth noticing it when we do.

Clifton Mark is a former academic with more interests than make sense in academia. He writes about philosophy, psychology, politics, and pastimes. If it matters to you, his PhD is in political theory. Find him @Clifton_Mark on Twitter.