environmental resources for the existing as well as the new offspring, when the mother is in
good health, and when her mate is of high quality.
A mother with minor (as opposed to major) PPD should furthermore not automatically lose
interest in all her children. According to PI theory, loss of interest should generally pertain only
to the reproductively least valuable offspring, i.e., either to the new infant or to one or more
existing children. While there will be exceptions to this—the father leaves after the birth of a
second child for example, making it impossible for the mother to raise any offspring—loss of
interest should not automatically apply to all children. This represents a potentially sensitive and
easy test of the defection hypothesis. If loss of interest in a new infant always implies loss of
interest in existing children, the defection hypothesis for minor PPD will be seriously
undermined. Reduced investment in existing children, mates, and other activities postpartum
clearly warrants further study.
Primiparous women with few future opportunities to conceive present another opportunity
to test the defection hypothesis. Older primiparae should be less likely to lose interest in the
infant, i.e. defect, since the current infant may be their only chance to reproduce. Possible
evidence against this prediction comes from the study of Kumar and Robson (1984): older
(30+) primiparae were more likely to experience PPD (p<.05). Several possibilities may
account for this result: 30 may be still too young for this effect to be noticeable, rates in these
primiparae may be a consequence of higher rates of depression among older women in the
general population, these rates may reflect higher rates of low viability offspring among older
women, or these cases of PPD may not have involved loss of interest in the infant.
Nevertheless, if this result is replicated for women with few future opportunities to conceive, if
the depression is found to be causally related to childbirth (rather than other life events or past
history), and if the depression involves loss of interest in the infant, it would weaken the
defection hypothesis.
Conclusion
Human reproductive effort is characterized by a very large degree of biparental care.
Because human infants require enormous amounts of investment, ancestral mothers needed to
carefully assess both the availability of support from the father and family members, and infant
viability before committing to several years of nursing and childcare. If a child was unlikely to
survive to reproductive age due to lack of sufficient investment or low viability, then it was in
the mother’s reproductive interest to defect from the costly childrearing venture. It is highly
unlikely that mothers will blindly invest in all infants without carefully (though perhaps
partially unconsciously) evaluating, at a minimum, levels of support and infant viability.
Consistent with these predictions of PI theory, there is compelling evidence that a perceived
lack of support, and problems with the pregnancy, delivery, or infant are correlated with PPD,
regardless of whether one assesses PPD by levels of depressive symptomology or clinical
diagnosis. Studies showing that prepartum measures of infant viability and perceived levels of
social support (when the mothers are not depressed) predict PPD support the view that infant
viability and low levels of social support are in fact etiological factors—they cause PPD. The
cognitive, affective, and behavioral manifestations of PPD characterize individuals who have
suffered a cost, experience that cost negatively, are motivated to reduce that cost, and, in fact,
act on their motivations. Mothers with PPD mother less. In sum, the evidence suggests that
PPD may be an adaptation that functions to inform mothers that they have suffered a
reproductive cost, and that successfully motivates them to reduce this cost by reducing or
eliminating maternal investment postpartum. These data for PPD strongly support the
27