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.
Mood is more complicated.
Because it is a continuum from high to low, we must look, not for a single situation, but for some varying aspect of the environment whose different levels require different behavioral strategies. A primary goal is discover what environmental variable is tracked by mood. The environmental variable that seems most likely to regulate mood is the perceived propitiousness of current circumstances. In propitious situations, small investments have a high likelihood of a large payoff. In unpropitious situations, any amount of effort is likely to be wasted.