And yet still, the belief in blue Mondays persists.

Several years ago, in another study, I examined expectations about mood and day of the week: two-thirds of the sample nominated Monday as the “worst” day of the week. Other research has confirmed that this sentiment is widespread, despite the fact that, well, we don’t really feel any gloomier on that day.

The question is, why? Why do we believe something that our own immediate experience indicates simply isn’t true?

As it turns out, the blue Monday mystery highlights a phenomenon familiar to behavioral scientists: that beliefs or judgments about experience can be at odds with actual experience. Indeed, the disconnection between beliefs and experience is common.

Vacations, for example, are viewed more pleasantly after they are over compared with how they were experienced at the time. And motorists who drive fancy cars report having more fun driving than those who own more modest vehicles, though in-car monitoring shows this isn’t the case. The same is often true in reverse as well: we remember pain or symptoms of illness at higher levels than real-time experience suggests, in part because we ignore symptom-free periods in between our aches and pains.

HOW do we make sense of these findings? The human brain has vast, but limited, capacities to store, retrieve and process information. Yet we are often confronted with questions that challenge these capacities. And this is often when the disconnect between belief and experience occurs. When information isn’t available for answering a question — say, when it did not make it into our memories in the first place — we use whatever information is available, even if it isn’t particularly relevant to the question at hand.