investment elsewhere5. On the other hand, the infant has just spent nine months inside the
mother, and the mother may be (unconsciously) privy to information about infant quality that is
not accessible to the husband or other family members, and thus be less inclined to provide the
levels of investment that others think she ought.
In ancestral environments, biparental care was a very high stakes game, and serious
conflicts of interest were undoubtedly frequent. Far from being a weakness, it is possible that
PPD may best be viewed as a potent, evolved strategy for leveraging the considerable power
mothers had over their own reproductive value and the value of their children once ‘nice’
strategies had failed.
Is PPD an honest signal of need?
Several researchers have suggested that depression is a costly, and therefore honest signal
(e.g., Price et al., 1994; Watson & Andrews, unpublished ms). When conflicts of interest exist
between senders and receivers of signals, an inherently costly signal provides some assurance to
the receiver that the signal is honest rather than deceptive (see, e.g., Grafen, 1990, for the
technical details of this argument; Zahavi & Zahavi, 1997). Price, et al. (1994) and Watson and
Andrews (unpublished ms) have both pointed out that depressed individuals are paying a large
cost by not pursuing reproductively ‘profitable’ opportunities, and that depression may
therefore be viewed as an honest signal. While Price, et al. view depression as a signal of
submission after losing a social competition, Watson and Andrews argue, among other things,
that depression is a signal of need, an idea first seriously pursued by Henderson and
collaborators (Henderson, 1974; Henderson, 1977; Henderson, 1978; Henderson, 1981;
Henderson, Byrne, Duncan-Jones, Scott & Adcock, 1980)6. Depression as an honest signal of
need is both consistent with, and complimentary to, the defection hypothesis. Workers going
on strike incur a cost by forfeiting their salary, and this cost does indeed signal something
positive about the legitimacy of their complaints.
The case of depression in general
As the editor of a recent volume on depression has noted about this affliction, “[d]espite a
great deal of thorough research there is no agreement concerning the etiology, symptomatology,
and treatment methods (Wolman, 1990).” Even diathesis-stress models that appear to well
characterize both PPD and depression in general (O'Hara, 1995) beg several questions, including
1) what types of events count as “stress” and why? and 2) why do stressful events provoke the
particular set of symptoms involved in depression as opposed to any other set of symptoms?
Why doesn’t stress provoke anger or fear or relief? Why psychomotor retardation, loss of
interest, loss of energy, etc.? An evolutionary approach is ideal for pursuing these levels of
explanation, and thus has much to offer the current debate and research on the definition and
nature of stress and the psychological symptoms it provokes. For the functional hypothesis
presented here, “stress” would be defined as evolutionarily costly events or situations like lack of
social support and infant problems. It should not apply to events that do not impose
evolutionarily significant costs. In this regard it is interesting to note that for the case of PPD,
5I thank Margo Wilson and Martin Daly for pointing this out.
6Note that Watson and Andrews conceptualize need as a set of socially imposed constraints on the pursuit of
fitness-enhancing activities, similar to the defection hypothesis, while Henderson, et al. view need as the low
quantity and quality of social relationships.
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