The Functions of Postpartum Depression

psychological pain hypothesis. Several symptoms of PPD, especially in its major form, do not
appear consistent with the psychological pain hypothesis, however. These symptoms have the
effect of preventing the mother from providing a wide range of benefits to either herself or to
others. Given that two major adaptive problems faced by mothers during the puerperium are to
either elicit investment from others or to avoid the imposition of social costs, and that credible
threats of defection from social contracts are an effective negotiating tactic, major depression
may thus be viewed as an adaptation to elicit greater investment or to reduce the mother’s
investment while avoiding social costs by making just such a credible threat of defection.

Although the data presented above may be consistent with hypotheses and theoretical
approaches to PPD not discussed in this paper, they do suggest that the defection hypothesis
deserves serious consideration as a possible explanation for certain aspects of this affliction.
Because many researchers argue that there is little to distinguish PPD from depression
occurring at other times (O’Hara & Zekoski, 1988; Watson, Elliott, Rugg & Brough, 1984;
Whiffen, 1991; Whiffen, 1992; Whiffen & Gotlib, 1993), any conclusion reached for PPD may
also be applicable to depression in general. The ability to defect from costly ventures has
obvious utility, and many researchers argue that this ability is one of the foundations of
cooperative behavior—cooperation cannot evolve if individuals have no means to renegotiate
or terminate costly relationships (see Axelrod & Dion, 1988 for a review). If depression is a
strategy that humans use to renegotiate or defect from costly social relationships, it would be
significant in understanding the evolution of cooperation.

The puerperium is an ideal venue for studying the evolution of social cognition since
parents predictably encounter costs and benefits that are both large and well defined from an
evolutionary perspective. Furthermore, the data described herein provide solid evidence that at
least one category of puerperal decision-making adheres closely to evolutionary expectations.

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