The frequency illusion (also known as the Baader–Meinhof phenomenon) is a cognitive bias in which a person notices a specific concept, word, or product more frequently after recently becoming aware of it.

The name "Baader–Meinhof phenomenon" was coined in 1994 by Terry Mullen in a letter to the St. Paul Pioneer Press. The letter describes how, after mentioning the name of the German militant group Baader–Meinhof once, he kept noticing it. This led to other readers sharing their own experiences of the phenomenon, leading it to gain recognition. It was not until 2005, when Stanford linguistics professor Arnold Zwicky wrote about this effect on his blog, that the name "frequency illusion" was coined.

Several possible causes behind frequency illusion have been put forth. However, the consensus seems to be that the main processes behind this illusion are other cognitive biases and attention-related effects that interact with frequency illusion. Zwicky considered this illusion a result of two psychological processes: selective attention and confirmation bias.

The main cause behind frequency illusion, and other related illusions and biases, seems to be selective attention. Selective attention refers to the process of selecting and focusing on selective objects while ignoring distractions. This means that people have the unconscious cognitive ability to filter for what they are focusing on.

Selective attention is always at play whenever frequency illusion occurs. Since selective attention directs focus to the information they are searching for, their experience of frequency illusion will also focus on the same stimuli. The process of frequency illusion is inseparable from selective attention, due to the cause-and-effect relationship between the two, so the "frequent" object, phrase, or idea has to be selective.

This means that a particularly triggering or emotive stimulus could catch someone's attention, possibly more than a mundane task they are preoccupied with.

Confirmation bias is a cognitive bias that always interacts with frequency illusion. This bias refers to the tendency of seeking evidence that confirms one's beliefs or hypotheses, while sometimes overlooking evidence to the contrary. Confirmation bias takes effect in the later stages of selective attention, when the individual has already started noticing the specific stimulus. By focusing on this specific stimulus, the individual notices it more, therefore confirming their suspicions of it occurring more frequently, even though in reality the frequency has not changed. In essence, confirmation bias occurs when the individual affected by frequency illusion starts looking for reassurance of this increased frequency, believing their theories to be confirmed as they focus only on the supporting evidence.

Recency illusion is another selective attention effect that tends to accompany frequency illusion. This illusion occurs when an individual notices something recently, leading them to be convinced that it originated recently as well. This phenomenon amplifies frequency illusion since it leads the person to become more aware of recent stimuli and increases the chances of them focusing on it in the near future. Similar to frequency illusion, recency illusion is also a result of selective attention, and can be overcome by fact-checking.