This paper by James Turner has been published by Erkenntnis. The abstract says:
“There are three main accounts of the proper function of the low mood system (LMS): the social risk theory, the disease theory, and the propitiousness theory. Adjudicating between these accounts has proven difficult, as there is little agreement in the literature about what a theory of the LMS’s proper function is supposed to explain. In this article, drawing upon influential work on the evolution of other affective systems, such as the disgust system and the fear system, I argue that a theory of the proper function of the low mood system should: (i) account for the reliable, distal causes and effects of the system’s activation, and (ii) explain how having a system that performed such a function increased fitness in ancestral environments. On this basis, I show that the proper function of the low mood system is to limit resource expenditure in relatively unpropitious circumstances, exactly as hypothesised by the propitiousness theory.”
You can read the full paper from here.