Psychological pain theorists have suggested that individuals who have recently suffered a
“social injury” should devote time to evaluating the causes of this injury before embarking on
new social ventures (Alexander, 1986; Nesse, 1991; Nesse & Williams, 1995; Thornhill &
Thornhill, 1990; Thornhill & Thornhill, 1989; Tooby & Cosmides, 1990). They argue that
‘lack of interest’ may function to prevent an individual from engaging in new social ventures
before fully evaluating the failure of the previous social venture. However, I do not find this
argument persuasive in accounting for the symptomology of major depression (a syndrome that
most evolutionary researchers on depression do not view as an adaptation in any case, e.g.,
XX). First of all, a diminished ability to think or concentrate and hypersomnia are precisely the
opposite of what one would expect if individuals were attempting to evaluate a recent social
failure—individuals should devote considerable thought to, and concentrate on, the failure, not
be prevented from doing so by depression. Second, while individuals would be expected to
devote additional time to evaluating a recent social failure at the expense of pursuing new social
ventures, they would not be expected to put their physical health at risk. Psychomotor
retardation, a marked loss of interest in virtually all activities, fatigue and loss of energy,
weight loss, and hypersomnia would have endangered the lives of individuals living in small,
hunter-gatherer groups by impeding their efforts to feed, care for, and protect themselves.
Indeed, it is difficult to see how a symptom like significant weight loss can have any utility vis-
а-vis evaluating a social failure.
Adaptations evolved because they solved recurring reproductive problems in ancestral
environments. In particular, psychological adaptations evolved to extract information from the
environment that was relevant to reproductive problems, and to then generate behaviors, that,
on average, solved these problems. Mothers with insufficient social support and/or a costly
infant faced two major problems: how to negotiate increased investment from others, or, how to
avoid the severe social costs they may have faced if they neglected or killed their offspring.
Just as mothers are unlikely to automatically invest in every newborn, fathers and family
members are unlikely to automatically invest either. New mothers wishing to raise their infant
may then face the problem of negotiating levels of investment from each of these interested
parties. If other members of the mother’s social environment have the potential to invest more,
the mother may be able to elicit more investment than she is currently receiving, making the
childrearing venture profitable. She should not immediately abandon the child without first
seeing if she can negotiate greater levels of investment.
I argue that major PPD may be a strategy to negotiate greater investment from the father and
kin, or to reduce the mother’s costs, by functioning somewhat like a labor strike (see Watson &
Andrews, unpublished ms, for a similar view). In a labor strike, workers withhold their own
labor in order to force management to either increase their wages and benefits, or reduce their
workload. Similarly, mothers with PPD may be withholding their investment in the new and
existing offspring, or, in cases of very severe depression, putting at risk their ability to invest in
future offspring by not taking care of themselves. This may force the father and kin to increase
their investment and/or allow the mothers to reduce their levels of investment.
Trivers’ theory of reciprocal altruism (1971), and later work on the evolution of cooperation
that it inspired (see Axelrod and Dion 1988 for a review), help provide a more general
formulation of the “labor strike” analogy. Like the relationship between a worker and her boss,
the human pair bond is a cooperative venture—both the mother and father agree to participate in
the mutually fitness enhancing endeavor of child rearing. As models of the evolution of
cooperation make clear, individuals will evolve to provide benefits to others only if they are free
to defect from these activities (e.g., quit) should their costs outweigh their benefits. If the costs
of child rearing outweigh the benefits, mothers (and fathers) will need to defect from this costly
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