This concludes part one of the paper. There is strong evidence that circumstances that would
have represented increased fitness costs to mothers in ancestral environments are etiological
factors for negative affect—sad or depressed mood—in modern mothers. Mother’s sad or
depressed mood may function analogously to physical pain to inform her that she is suffering or
has recently suffered a serious fitness cost, and, as will be explained in the next section, motivate
her to reduce this cost.
The defection hypothesis, part two: reduce costs
This section presents a functional hypothesis for the second major symptom of PPD: loss of
interest. If a mother’s sad or depressed mood informs her that she has suffered a reproductive
cost, then it is possible that she will act to reduce this cost in order to allocate her finite
resources to more beneficial ventures, consistent with PI theory. We should therefore expect
mothers with PPD to frequently experience a loss of interest in the infant, and, indeed, this is a
prominent symptom of PPD (APA, 1994; Beck, 1992; Beck, 1996b; Campbell et al., 1992).
Not only do mothers with PPD often lose interest in their child, they frequently have thoughts
of harming them (Jennings et al., in press). Loss of interest in the infant is not the only strategy
mothers have to reduce their costs, however. Conceivably, they may decide to direct a greater
proportion of their time and energy towards their infant by losing interest in other activities like
caring for older children, caring for other family members, doing household chores, working,
etc. Anecdotal evidence for this hypothesis is found in an interview with a depressed mother
(Beck 1996, p. 102):
When I was going through the depression real bad, I pushed away my daughter and
my husband. It was like I just wanted to take care of the baby and I didn’t want to
take care of anyone else. I could only deal with one person, and the rest of you
should go away, ‘cause I can’t deal with the rest of it.
Beck notes that mothers in her study were unable to cope with more than one child at a
time, resented their older children, and pushed them away. Because older children are more
likely to survive with reduced maternal investment than are newborns, mothers with a viable
offspring or an offspring with reversible health problems, may opt to focus their efforts on the
baby if they can count on the husband or relatives taking care of the other children. Similarly,
mothers may have to reduce their investment in their husbands in order to focus all their effort
on the newborn. Thus, when mothers have a healthy baby and a supportive mate, but also have
significant and time consuming responsibilities like caring for other young children, family
members, their mate, or producing income, etc., they may defect from these other
responsibilities in order to invest in the newborn. Excessive non-infant related responsibilities
may also predict PPD, with loss of interest applying to activities other than infant care.
While PPD may be an adaptation to defect from any costly activity during the puerperium,
a focus on loss of interest in the infant is warranted since this symptom is likely to have the
greatest detrimental effect on child development, a serious negative consequence of PPD (e.g.,
Murray, 1992; Whiffen & Gotlib, 1989).
Many women with PPD are unable to feel any joy or love in taking care of their infants, and
often have obsessional thoughts about harming them (APA, 1994; Beck, 1992; Beck, 1996b;
Jennings et al., in press), symptoms which are clearly consistent with a desire to defect.
Similarly, “negative emotions while with the baby” are significantly correlated with PPD
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