Because each of these hypotheses involves either an aspect of a mother’s decision to invest in
or defect from childrearing, or her attempt to negotiate larger levels of support by threatening to
defect from childrearing, I will refer to them collectively as the defection hypothesis for PPD.
The defection hypothesis, its theoretical foundations, and supporting data will be presented in
detail in the following sections. These data are not sufficient to prove the defection hypothesis,
however. Other interpretations of the data are possible, and I consider it well beyond the scope
of the paper to analyze these other interpretations (there is no consensus on the correct
theoretical approach to PPD; see Affonso (1984), Cutrona (1982), and Hopkins (1984) for brief
reviews of psychodynamic, personality, cognitive-behavioral, and biophysical theories of PPD.
See Cramer (1993), Collins et al. (1993), Cutrona (1983), Cutrona and Troutman (1986), Gotlib
et al. (1991), O’Hara et al. (1982), and O’Hara et al. (1984) for experimental tests of particular
theories of PPD).
Theoretical Foundations
Parental investment theory
The close association of PPD with child bearing and rearing suggests that the application of
parental investment (PI) theory may be quite fruitful (see, e.g., Clutton-Brock 1991). PI theory,
an aspect of life-history theory, provides the evolutionary framework for nearly 20 years of
research into parental investment in offspring for both humans (e.g., Betzig, Borgerhoff Mulder
& Turke, 1988; Blurton Jones, 1989; Chisholm, 1993; Dickemann, 1979; Dickemann, 1981;
Draper & Harpending, 1982; Hagen, 1996; Hagen, 1998c; Hagen, submitted; Haig, 1993;
Hames, 1996; Hartung, 1982; Hartung, 1985; Hill & Kaplan, 1988; Hrdy, 1992; Lampert &
Friedman, 1992; Voland, 1984) and other species (the literature is huge; for recent syntheses see
Clutton-Brock (1991), Roff (1992) and Stearns 1992).
Both PI theory and life-history theory (of which PI theory is a part) form the basis of this
functional analysis of PPD. To briefly review, life-history theory posits that in order to have
left descendants, the ancestors of any species must have solved the problems of survival,
growth, development on the one hand, and reproduction on the other. Because each of these
problems is characterized by unique difficulties, and because time, energy, and resources are
finite, organisms must optimally allocate these commodities between somatic effort (growth,
development, and maintenance of the organism), and reproductive effort (producing offspring
who themselves survive to reproductive age).
Reproductive effort, in turn, should be optimally allocated between mating effort (locating
and acquiring a mate), and parenting effort (e.g., gestation and raising of offspring)—what I
have here termed parental investment in order to be consistent with existing literature (see
Clutton-Brock 1991, p. 8, for a discussion of terminology). PI theory focuses on those aspects
of an organism’s life-history that are specifically involved with producing and raising offspring.
Life history theorists assume that the physiological and behavioral characteristics of
organisms represent an approximate solution to the problem of optimizing the allocation of time,
energy, and resources between somatic, mating, and parenting effort, with the particular solution
depending on the organism’s environmental niche as well as its evolutionary history. In general,
effort allocated to reproduction will decrease an organism’s ability to survive, grow, and
develop, whereas, conversely, effort allocated to survival, growth, and development will
decrease reproduction. Similarly, effort allocated to finding a mate will decrease an organism’s
ability to invest in offspring, whereas effort invested in offspring will reduce an organism’s
ability to acquire a mate. If parental investment can only occur at the expense of somatic or