subject to special dietary restrictions during lactation, and must provide virtually all of the
infant’s nutrition for at least the first six months postpartum (Wood, 1994). Lactation will
occur at the expense of maternal stores, and the net effect of these differences is that the
nutritional status of women in traditional societies often declines sharply over the course of
lactation, a phenomenon referred to as maternal depletion syndrome (Bongaarts & Delgado,
1979; Miller & Huss-Ashmore, 1989). Nursing an infant can seriously impact the mother’s
health.
The postpartum period is clearly a very expensive one for the nursing mother, much more
so than pregnancy. The decision to invest in the infant is consequently of critical importance,
and a necessary functional component of the mother’s decision-making process is information
on whether her costs are in fact exceeding her benefits.
Many researchers have attempted to identify functions for psychological distress, neuroses,
and depression which would compensate for their obvious costs (Alexander, 1986; Birtchnell,
1993; Gardner, 1982; Gilbert, 1989; Henderson, 1974; Henderson, 1981; McGuire & Troisi,
1998; Nesse, 1991; Nesse & Williams, 1995; Price, Sloman, Gardner, Gilbert & Rohde, 1994;
Slavin & Kriegman, 1992; Thornhill & Thornhill, 1990; Thornhill & Thornhill, 1989; Watson
& Andrews, unpublished ms; Wenegrat, 1995). Virtually all propose functions involving an
adaptive response to varied interpersonal problems. In particular, several of these researchers
have proposed that whereas physical pain functions to inform individuals that they have
suffered a bodily injury, psychological pain informs individuals that they have suffered a non-
bodily injury (sometimes referred to as a “social injury”) motivating them to cease activities
which would further this injury, as well as to avoid any future situations which would also
result in injury (Alexander, 1986; Nesse, 1991; Nesse & Williams, 1995; Thornhill &
Thornhill, 1990; Thornhill & Thornhill, 1989; Tooby & Cosmides, 1990). A non-bodily injury
is any circumstance, usually involving relationships with others, that was reliably associated
with a reduction in reproductive fitness over evolutionary time, e.g., the death of children and
relatives, loss of status, etc.
An important symptom of PPD is a sad or depressed mood (APA, 1994). The first and most
strongly supported functional hypothesis for PPD offered here is that sad or depressed mood is
in fact information to the mother that she is suffering (or has recently suffered) circumstances
that were reliably associated with net reproductive fitness costs over evolutionary time.
Failure to offset puerperal costs: hypothesized etiological factors for PPD
If PPD functions, in part, to inform mothers that they are suffering or have suffered
circumstances that were reliably associated with net reproductive fitness costs, then these
circumstances should be important etiological factors for PPD. New mothers have just invested
nine months in the new offspring, and they need to evaluate their decision to get pregnant,
decisions made during pregnancy, and whether to continue to invest in the offspring in light of
the following circumstances that would have either substantially increased their costs or
reduced their benefits (see Mann, 1992 for a similar list):
1. There is insufficient investment from the father or others to successfully raise the
offspring.
2. There are problems with pregnancy, birth, or with the infant that indicate that this
offspring may have low viability, that is, is unlikely to survive to reproductive age.
3. Environmental conditions are poor for raising an offspring (e.g., harsh winter,
insufficient resources).