Have you ever bought a new shirt and then suddenly noticed everyone around you wearing the same one? Or learned a new word and started hearing it more often? Has a friend told you she was pregnant and then it seemed like everyone else was pregnant, too?

If so, congratulations: you are human after all. What you've just experienced may be a frequency illusion, or "the Baader-Meinhof phenomenon."

What is the frequency illusion?

A frequency illusion is a cognitive bias , which takes place in the mind and affects the way we perceive the world around us.

Arnold Zwicky, a linguistics professor at Stanford University in the US, coined the term "frequency illusion" in 2005.

The idea was that we become more aware of things when we learn about them for the first time and that that can cause us to think that they are happening more often than they are in reality.

For example, if you buy a new red car, you may start seeing red cars more frequently and think that more people are buying red cars, or that red cars are trendy, when in reality it is possible that the number of red cars hasn't changed at all.

If we counted the number of red cars, or checked official statistics for car sales, we may indeed find that there are more red cars on the roads. But we're talking about the perception of facts here, rather than the facts themselves. 

And that's important because such perceptions, or frequency illusions, can affect the way we think and make decisions in our daily lives. We may, for example, overestimate crime rates when crimes are covered more frequently in the media.

Why does the frequency illusion happen?

"We are evolutionarily trained to seek explanations," said mathematics professor David Hand from Imperial College London.

Seeking explanations can be helpful if you want to learn new things, but it can also lead to our brain forming biases or even prejudicial opinions.

If, for instance, those crime reports we mentioned earlier focus on a certain type of crime — say, knife crimes among young people — and the reports are all from a particular community, you might be led to believe that those crimes only happen there among those people. 

We tend to like such easy conclusions because they trigger our brain's reward center. It feels good to be able to explain a thing, even if only to ourselves, and then have that explanation confirmed, doesn't it?  

The problem is that we may then become overly confident about what we say, even if we know very few facts about the matter.  

Confidence is not a strict measure of whether we are right or wrong to think a certain way. If we believe or feel our version of a story is confirmed, we are likely to be more confident about it even if we've got it all wrong — or we're missing some of the facts, or context.

Psychologist Daniel Kahneman says this can create an illusion of validity, which can make it harder for us to admit we might be wrong. 

Two underlying mental processes that cause a bias

The frequency illusion is the result of two psychological processes. 

First, there's selective attention. Selective attention allows us to focus on what is currently important to us. That could be our new car, a new topic or a natural disaster we saw on the news, such as a tornado or an earthquake. 

Selective attention is important for our ability to learn and our memory functions.

We need selective attention to focus on tasks or pay attention to what someone is telling us in a conversation, while ignoring all the other signals around us, such as planes over head, birds chirping, smells, or any other distraction. Without selective attention, we wouldn't be able to do much.  

Second, there's confirmation bias. Confirmation bias is closely related to the illusion of validity. Because we notice red cars more, we feel more confident that red cars are more common, and that further confirms our belief that there are more red cars.

Why is the frequency illusion also known as the Baader-Meinhof phenomenon?

The frequency illusion is often referred to as the Baader-Meinhof phenomenon — not because the German terrorist group had had anything to do with it, but because of a discussion on a US newspaper's bulletin board in 1994.  

One of the newspaper's readers, a man called Terry Mullen, posted a comment, saying that he had recently learnt about the Baader-Meinhof for the first time and that he had then started hearing the term more frequently.

Other readers on the online forum responded by sharing similar stories, also involving the name Baader-Meinhof. A discussion grew from that and the name stuck, so people now often talk about the Baader-Meinhof phenomenon when they mean the frequency illusion. 

Mental shortcuts to frequency illusion

The frequency illusion is a type of a mental shortcut called a heuristic.

In a paper published by Kahneman, and a colleague called Amos Tversky, in the US journal Science in 1974, they wrote that heuristics help us make decisions and accomplish tasks quicker.

Heuristics can be helpful when we don't have all the information we might need to make a fully rational decision but need to progress — a bit like using a rule of thumb.  

There are different types of heuristics, such as the availability heuristic, where we make decisions based on the information most readily available to us in our minds.

And there is the representativeness heuristic, where we judge something based on our preconceptions — or stereotypical ideas, prejudices. 

We need heuristics to get by in everyday life. But they are not perfect, said Sekoul Krastev, a decision scientist and managing director at The Decision Lab, a Canadian think tank.

"Because they're imperfect shortcuts to solving problems, they do sometimes lead us to make mistakes," Krastev said. 

Check out the bat and ball problem: A bat and ball together cost $1.10 (ca. €1.04). The bat costs $1 more than the ball. How much does the ball cost? Most people intuitively say 10 cents, but the correct answer is 5 cents. Do you know why?

Edited by: Zulfikar Abbany