problems: increase the levels of investment of others, or reduce the social costs of defecting from
childrearing.
The defection hypothesis, part three: negotiation
Why, if caring for a newborn or engaging in other activities is not in a mother’s
reproductive interest, does she not simply stop engaging in the offending task? Why does she
experience the distress of depression with its numerous attendant symptoms? Since a
significant fraction of depressions postpartum are minor (Whiffen, 1992), most mothers with
PPD may in fact only experience a few symptoms consistent with “low mood.” The
psychological pain hypothesis well accounts for these cases. A significant fraction of cases of
PPD are major depressions, however. For example, in their study of 1033 new mothers,
Campbell and Cohn, 1991 found that among women with clinically diagnosed depression, 38%
had major depression, 31% had probable major depression, and 31% had minor depression.
The Diagnostic and Statistical Manual (DSM) IV (APA, 1994, p. 386) lists the following
symptoms of a major depressive episode (a functional hypothesis has been, or will be offered
for symptoms in bold):
1. sad or depressed affect
2. marked loss of interest in virtually all activities
3. significant weight loss or gain
4. hypersomnia or insomnia
5. psychomotor retardation or agitation
6. fatigue or loss of energy
7. feelings of worthlessness or guilt
8. diminished ability to think or concentrate
9. recurrent thoughts of death
So far, this paper has presented a functional argument for symptom one, and a partial
argument for symptom two—a loss of interest in particular, as opposed to virtually all,
activities. These two symptoms would have delivered benefits to ancestral mothers with a low
viability infant or who lacked sufficient social support; when they appear in isolation, or with
only one or two other symptoms, they are consistent with a minor depression. The data
strongly support the view that minor postpartum depression is an adaptation.
If these other symptoms of major depression are adaptations, then they must have delivered
benefits to ancestral mothers, yet they seem designed to prevent the acquisition of benefits. A
marked loss of interest in virtually all activities, significant weight loss, hypersomnia,
psychomotor retardation, fatigue or loss of energy, and a diminished ability to think or
concentrate would all have impeded ancestral mothers from engaging in critical, beneficial
activities unrelated to childrearing, such as food gathering and consumption, buffering food
shortages, personal hygiene, avoiding environmental hazards, information gathering, helping
relatives and friends, etc. The costs of these symptoms could easily have outweighed the
benefits of the first two symptoms, seriously undermining the argument that major PPD may be
an adaptation. The hypothesis I will explore here is that major depression prevents individuals
from acquiring benefits by design. The extraordinarily distressing symptoms of major
depression, almost universally interpreted as pathological, may in fact be functional.
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