Sometimes you know there's just the right word for something, but your brain can't find it. That frustrating feeling is called the tip-of-the-tongue (TOT) state—and for decades psychologists assumed it was caused by a partial recollection of the answer. But new research suggests this experience may be largely an illusion. Being sure you know something doesn't mean you actually do.

In a series of experiments published in the Journal of Experimental Psychology: General, college students attempted to answer 80 general knowledge questions with one-word answers. If they didn't provide a correct answer, they were asked if they felt like the answer was on the tip of their tongue and to provide partial information such as its first letter, its number of syllables, or what it sounded like. The team found that people in a TOT state were more likely to volunteer partial information—doing so five times as often in one experiment.

But that information tended to be wrong. Guesses at sounds and syllable counts were no more likely to be right in a TOT state than otherwise. Averaged across several experiments, first-letter guesses were only slightly more likely to match the correct answer (roughly 11 versus 8 percent). Yet participants said they thought their guess was correct 58 percent of the time while in TOT states versus 7 percent otherwise.

Previous research has shown that TOT states are not completely illusory—people better recognize correct multiple-choice answers following such states (55 versus 42 percent)—but this work joins burgeoning research indicating that we can't fully trust them. The evidence suggests that instead of partial recollection leading to a TOT state, a reverse process may be taking place: something triggers the feeling, which then motivates people to search their memories and to retrieve partial (and usually incorrect) information.

The new study “demystifies this state,” says Columbia University psychologist Janet Metcalfe, whose own research has separately suggested that TOT feelings correlate with curiosity to learn the real answer. But the mechanism behind the TOT experience remains a mystery. “One possibility is that people detect familiarity with the question itself,” says the study's senior author Anne Cleary, a psychologist at Colorado State University. “It may be signaling: ‘something relevant is here in memory—let's do a search.'”

Cleary relates TOT to a similar state: déjà vu, which is especially common in people with certain neurological disorders such as epilepsy. In both cases, a compelling feeling of familiarity occurs, and we try to make sense of it by telling ourselves we must have seen or learned something before. Confabulation, she says, is more common than we realize.