Being moody may not be such a bad thing, according to a new theory published in the journal Trends in Cognitive Sciences.

"It has long been thought that happiness induces a 'rosy' perspective, whereas a depressed mood engenders negative judgements," researchers from the University College London said. "More recently, researchers have used computational methods in laboratory experiments to precisely quantify the effects of emotional state on behavior." Put it another way: experiences may affect mood, which in turn affect subsequent experiences.

With this idea, researchers propose people's experiences are colored by their mood, and their expectations reflect both the reward outcomes and changes in the overall availability of reward in the environment.

"This effect of mood should be useful whenever different sources of reward are interconnected or possess an underlying momentum," lead study author Eran Eldar said in a press release. "That may often be the case in the natural as well as in the modern world, as successes in acquiring skills, material resources, social status, and even mating partners may all affect one another."

Eldar and his team give the example of increased rainfall or sunshine causing more fruit to grow in trees. In this scenario, it makes more sense to expect there will be fruit in all related trees, and not just this individual tree. And "if fruit becomes more abundant in all trees, a foraging animal will be positively surprised multiple times as it visits adjacent trees and, as a result, its mood will improve."

"Through the existence of mood, as an animal learns from experience, its expectations come to reflect not only the reward associated with each particular state (e.g., each tree), but also recent overall changes in the availability of reward in its environment," researchers explained. In this way, they said learning can somewhat account for the impact of multiple environmental factors without having to directly consider the number of factors or how much of an effect they'll have.

More fruit may also be an indicator that "spring is coming," and in such a case, a positive mood would represent the belief that positive events will soon occur. Similarly, researchers explained if reward availability decreases in an environment (winter is coming), then a negative mood will lead to rewards being perceived as less good than they actually are. Expectations will catch up with the declining reward, and behavior will soon adjust accordingly (e.g., hibernation).

Eldar added that negative moods may have the potential to become self-fulfilling prophecies. He suggested if a negative mood persists, it may cause "a person to perceive many subsequent outcomes as worse than they really are, leading to a downward the onset of a depressive episode."

He continued: "We think that this novel approach may help reveal what predisposes particular individuals to bipolar disorder and depression."

It's worth reiterating that this is just one group of researchers' opinion. Basically, they have outlined a "normative perspective on mood," where mood changes based on the pattern of changes in reward. It suggests different ways in which mood function might be disrupted and possibly contributes to the development of depression and mood instability.

"Being moody at times may be a small price to pay for the ability to adapt quickly when facing momentous environmental changes," the researchers concluded.

Source: Eldar E, Rutledge RB, Dolan RJ, Niv Y. Mood as Representation of Momentum. Trends in Cognitive Sciences. 2015.