The Functions of Postpartum Depression

Table 3: Poor neonatal outcome is associated with “minor” pregnancy and delivery problems. These problems
are strongly associated with PPD (table 2 and text).

Considering that ancestral mothers didn’t have access to modern medical technology,
pregnancy and delivery problems would have been excellent predictors of lowered infant
viability, even in the absence of overt infant defects. Mothers with pregnancy or delivery
problems should therefore be more likely to consider defecting from the childrearing venture
than those without such problems.

If infants have health problems that would have been reliably reversible in ancestral
environments, e.g., moderately low birth weight or mild infections, and the resources to deal
with such problems, they should increase rather than decrease their investment in the offspring.
See Mann (1992) for a discussion of circumstances that should lead to an increase in maternal
investment in the infant. Even when mothers have healthy, happy babies and plenty of social
support, there may not be enough resources to successfully raise them, however. Assuming
adequate fat reserves, lactation still requires an additional 500 kcals/day (Worthington-Roberts
et al., 1985). If food is scarce, mothers may not be able to safely provide this resource to new
offspring without jeopardizing their own health or the health of existing children.

Finally, it is possible that higher quality long-term mates are available, even if the mother’s
current mate is willing and able to invest. Potential mates may be healthier, or have access to
significantly greater resources than the current mate, for example. Logically, “mate quality” is
distinct from “investment.” However, a father’s “mate quality” includes his ability and
willingness to invest in the mother and offspring. Because I am aware of no studies that
correlate PPD with the availability of potential mates that are healthier or have greater
resources, mate quality will, for the purposes of this paper, be considered synonymous with
ability and willingness to invest. Prediction 4c then reduces to prediction 1.

Given that each of the above factors should significantly impact mothers’ decisions to invest
in their infants, they should obviously also be significant etiological factors for affective states
that inform those decisions—each of these factors should predict PPD. As will be discussed
below, the evidence that factors one and two predict PPD is excellent; the evidence that factors
three and four predict PPD is currently limited.

Prevalence of postpartum depression vs. non-postpartum depression: a caveat

The defection hypothesis requires that pregnancy and childbirth are causal factors for PPD.
Given that the base rates for depression and depressive symptoms are high among women in
community samples, it is not clear that depressions occurring postpartum are anything other
than coincidentally related to pregnancy and childbirth, an important issue that is surprisingly
understudied.

Whiffen (1992) found the overall rate for PPD of 13.0% to be approximately double the
community rate for non-postpartum major and minor depression . The comparison of these

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