One function is to adjust the timing of effort. When investments are not paying off, it is wise to stop investing in order to conserve efforts for a later time. Less well appreciated are the benefits that high mood offers by increasing investments at times of high payoff. The other motivational function of mood is to regulate the locus of investment. If a strategy is not paying off, or if a goal seems unattainable, it is wise not only to conserve resources for a more propitious time, but also to reconsider the viability of the strategy.
Behaviorists propose a similar function in describing reactions to decreased reinforcement.
These four functions of mood are all correct, in part, but none is sufficient. An explicitly evolutionary approach suggests a broad hypothesis that integrates the functions of mood. Like other emotions, high and low mood are behavioral subroutines, specialized states that have been shaped to increase fitness in certain situations. An evolutionary explanation of an emotion does not just consist of describing its functions. The characteristics of fear, for example, are useful in situations that are dangerous.