Worldwide poverty has dropped dramatically, we are in the Long Peace when it comes to war, the old cycles of famine boom and bust have leveled off, science has made it possible for everyone to live better for longer and spend less on basic necessities.

The world continues to improve. Why, then, do polls consistently show that people believe otherwise? The answer may lie in a phenomenon called "prevalence induced concept change."

That means that each generation feels the need to 'live in important times.' If things are good, then a poorly written article can be spread Twitter as a reason only corporations should be in science media. As the prevalence of a problem is reduced, humans redefine the problem itself.As the problem becomes smaller, people's conceptualizations of that problem become larger, which can lead them to miss the fact that they've solved it.

Activists leverage this psychology quite well. Things are never good, there is always someone new to sue and you need to send them money right now or we are doomed. The phenomenon isn't limited to large, seemingly intractable social issues. In several experiments described a recent paper, it emerged even when participants were asked to look for blue dots. "We had volunteers look at thousands of dots on a computer screen one at a time and decide if each was or was not blue," said co-author Daniel Gilbert, Professor of Psychology at Harvard. "When we lowered the prevalence of blue dots, and what we found was that our participants began to classify as blue dots they had previously classified as purple."

Prevalence-induced concept change isn't always a bad thing. An emergency room in a small town is much different than doing triage on a battlefield.  In combat, a broken arm will have lesser priority than gunshot wounds, whereas in an ER with no gunshot victims, a doctor won't hold have the same "needing immediate attention" tell the broken arm to wait Definition do change based on this new context.

In other cases, however, prevalence-induced concept change can be a problem.

"Nobody thinks a radiologist should change his definition of what constitutes a tumor and continue to find them even when they're gone," Gilbert said. "That's a case in which you really must be able to know when your work is done. You should be able to see that the prevalence of tumors has gone to zero and call it a day. Our studies simply suggest that this isn't an easy thing to do. Our definitions of concepts seem to expand whether we want them to or not.

"Expanding one's definition of a problem may be seen by some as evidence of political correctness run amuck,. They will argue that reducing the prevalence of discrimination, for example, will simply cause us to start calling more behaviors discriminatory. Others will see the expansion of concepts as an increase in social sensitivity, as we become aware of problems that we previously failed to recognize." 

"Our studies take no position on this," he added. "There are clearly times in life when our definitions should be held constant, and there are clearly times when they should be expanded. Our experiments simply show that when we are in the former circumstance, we often act as though we are in the latter."

Ultimately, Gilbert said, these studies suggests that there may be a need for institutional mechanisms to guard against the prevalence-induced concept change.