solved these problems. It will be argued here that PPD shows evidence of having been
designed by natural selection to solve three important problems of the puerperium, and is
therefore not an illness, but an adaptation.
The suggestion that PPD is a functional component of human reproductive decision-making
was first made by Daly and Wilson (1988), and Wilson and Daly (1994), who have published
extensively on parental cognition in evolutionary perspective (Daly, 1987; Daly, 1995; Daly,
1980). In particular, Wilson and Daly (1994) pointed out that both lack of social support and
infant problems were associated with PPD, and that, in accord with evolutionary expectations,
PPD disinclined mothers to invest in their offspring. This functional hypothesis for PPD has
been further elaborated by Hagen (1998; 1998; 1996; 1998), and Thornhill and Furlow (1998),
and will be explored in detail here.
Before proceeding, it is important to point out that adaptations can operate even though
individuals may have no conscious awareness of their function. None of our ancestors had any
awareness or understanding of the function of fever, for example. In an attempt to avoid
lengthy and awkward sentences, however, I use the words “decide” and “should” to indicate the
operation of an adaptation, with no conscious or moral intent implied. For example, the phrase
“mothers decide to defect from their infants when...” is shorthand for “a maternal psychological
adaptation to defect from the infant is activated when....” Similarly, the phrase “mothers should
defect from their infants when...” is intended to be shorthand for “mothers who defected from
their infants when..., would have had, on average, a greater number of descendants than those
who did not defect.”
The defection hypothesis for PPD, part one: association of negative affect with
correlates of net fitness costs
This section will specify in greater detail some of the adaptive problems faced by ancestral
human mothers, and will propose one deceptively simple but very important psychological
function that would have been necessary to address these problems, namely, that ancestral
mothers needed to know when their costs were exceeding their benefits. PPD may, in part,
assist the mother in determining whether she is suffering (or has suffered) circumstances that
were associated with net fitness costs over evolutionary time. The other two components of the
defection hypothesis, that PPD may function to reduce investment by the mother or increase the
investment of others, will be discussed in later sections.
Costs of the puerperium
The major benefit of bearing an offspring—a reproductively successful child—involves a
substantial investment from the mother. The time, energy, and resources required to
successfully rear human infants are enormous, a consequence of the considerable degree of
brain growth and development that occurs postpartum (Lancaster, 1986). Human offspring are
unable to fend for themselves, requiring mothers to provide several years of direct care. In
addition to direct care, mothers in preindustrial populations provide all the fuel for brain
development. Breast feeding is metabolically expensive, and the energetic costs of lactation are
actually greater than the energetic costs of pregnancy (Worthington-Roberts, Vermeersch &
Williams, 1985). The 36,000 kcal of fat that a female may have stored under ideal conditions is
sufficient to provide only about one-third of the energy required to support four to five months
of lactation (Widdowson, 1976). The typical woman in a preindustrial setting is unlikely to
conform to this western ideal, however. She often begins pregnancy with lower nutritional
reserves, gains little if any fat during pregnancy, may lactate for more than three years, may be