The bright side of being blue: Depression as an adaptation for analyzing complex problems
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2734449/pdf/nihms129978.pdf

In this paper, we explain the impairments associated with depression, cognitive and otherwise,
by hypothesizing that depression is an evolved stress response mechanism. The hypothesis
consists of a series of claims, depicted diagrammatically in Figure 1, of which we now give an
overview, and which are discussed in greater detail below.

When selection causes a trait to evolve, it does so because the trait has a gene-propagating
effect, which is called the trait’s function (Andrews, Gangestad, & Matthews, 2002a; Thornhill,
1990; G. C. Williams, 1966). The cumulative effects of selection for genes that promote a
specific effect often leaves the tell-tale signs of its workings on a trait, and such evidence can
be used to make inferences about past selection pressures (Andrews et al., 2002a; Thornhill,
1990; G. C. Williams, 1966). When a trait has features that proficiently promote a specific
effect, that will go a long way to demonstrating that the effect is an evolved function of the
trait because it is highly unlikely that chance processes could be completely responsible for
the trait’s features (Andrews et al., 2002a; Thornhill, 1990; G. C. Williams, 1966).

One effect of sad or depressed mood is to promote an analytical reasoning style in which greater
attention is paid to detail and information is processed more slowly, methodically, thoroughly,
and in smaller chunks (Ambady & Gray, 2002; Edwards & Weary, 1993; Forgas, 1998; Gasper,
2004; Gasper & Clore, 2002; Schwarz, 1990; Schwarz & Bless, 1991; Yost & Weary, 1996).
Conversely, positive mood states promote heuristic or creative processing (Ambady & Gray,
2002; Isen, Daubman, & Nowicki, 1987). The second claim is that depression coordinates a
suite of changes in body systems that promote rumination, the evolved function of which is to
analyze the triggering problem. While analysis is used in science and many areas of modern
life, this claim proposes that it is part of the evolved human cognitive repertoire.

Analysis is time consuming and requires sustained processing, so it is susceptible to disruption,
which interferes with problem-solving. Depression induces changes in body systems,
producing effects that facilitate analytical rumination by reducing disruption (listed as body
system changes and facilitative effects in Figure 1).

Specifically, depressed affect:
(1) activates
neurological mechanisms that promote attentional control, which gives problem-related
information prioritized access to limited processing resources and makes depressive
rumination intrusive, persistent, resistant to distraction, and difficult to suppress;
(2) induces anhedonia, which reduces the desire to think about and engage in hedonic activities that could
disrupt problem-related processing; and
(3) promotes psychomotor changes that reduce
exposure to stimuli that could disrupt processing (e.g., desire for social isolation, loss of
appetite).