them either. This may ‘short-circuit’ cheater detection mechanisms, thereby reducing the
likelihood of being punished, a possibility that probably requires some constraint on the
evolution of cheater-detection mechanisms.
Although data supporting a negotiating function for PPD are few, they do uphold the main
outlines of the hypothesis. For example, women who report that their partners are
overcontrolling in the marriage have an elevated risk of PPD, especially when levels of
investment are low (Boyce, Hickie & Parker, 1991; Schweitzer, Logan & Strassberg, 1992). In
other words, when husbands attempt to control their wives’ actions but don’t provide
investment, their wives are especially likely to experience depression, as predicted by the
defection hypothesis. Furthermore, family members of women with PPD report that they are
more attentive to the mother’s needs as a consequence of her depression, and that they have
assumed many of her responsibilities. They also report that their increased investment is a
considerable burden and that arguments are common, suggesting conflict and negotiation over
who should do what (Boath, Pryce & Cox, 1998). Finally, as predicted, higher levels of
spousal support are associated with recovery from PPD (Campbell et al., 1992).
This functional defection hypothesis does not account for a few symptoms, in particular
significant weight gain (energy storage for rough times ahead?), insomnia (spend time thinking
about critical problems instead of sleeping?), and psychomotor agitation (?). Feelings of
worthlessness are consistent with low investment by others, or with the unwillingness of others
to renegotiate social contracts. Feelings of guilt are consistent with a desire to change or defect
from one’s existing social contracts. Suicidal ideation may represent the ultimate threat of
defection. That is, it, too, may be a strategy to extort those who benefit from their association
with the afflicted individual (Watson & Andrews, unpublished ms). While an evolutionary
analysis of these remaining symptoms, and, indeed, a careful test of part three of the defection
hypothesis, is beyond the scope of this paper, I hope I have provided at least the reasonable
expectation that such an analysis may be fruitful, and that the hypothesis of depression as a
credible threat of defection merits consideration. This proposed negotiating function for major
PPD does not replace the psychological pain hypothesis but in fact requires it. In order to
negotiate by threatening to defect, mothers still need to know when their costs are exceeding
their benefits, and they still need to be motivated to reduce their costs.
Why not be nice?
The defection hypothesis does not imply that mothers are precluded from using ‘nice’
strategies for negotiating increased investment. If they have received cues that such strategies
are likely to be successful, mothers may just ask for increased investment, perhaps in exchange
for concessions in other areas or for promising to provide future benefits, or perhaps because
the welfare of the child is at stake. A threat of defection should only be employed when there
is a serious conflict between the degree of investment that the mother feels she needs, and the
levels that the father and/or other family members are willing to provide—workers can be nice,
work hard, and still not make enough money.
Serious conflicts of this nature will not be uncommon. For most couples, the period of
cohabitation prior to pregnancy involves negotiating the terms of biparental care: who is
expected to do what for the family. Once the baby is born and the mother is nursing and unable
to pursue other mating opportunities, the father may be tempted to unilaterally change the terms
of the mateship by, for example, seeking other mates, especially if he has doubts about the
paternity of the newborn or if new mating opportunities have presented themselves. Mothers
are therefore expected to be especially sensitive to cues that the father intends to divert his
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