Evolutionary factors in postpartum depression

Evolutionary approaches to parental care (e.g., parental investment theory, parent-offspring conflict, the Trivers-Willard hypothesis, life-history theory) suggest that parents (human and non-human) will not automatically invest in all offspring, and will reduce or eliminate investment in their offspring when the costs outweigh the benefits. Reduced care, abandonment, and killing of offspring have been documented in a wide range of species. In many bird species, for example, both pre- and post-hatching abandonment of broods is common (Ackerman et al., 2003; Cezilly, 1993; Gendron and Clark, 2000; Trivers, 1972).

Human infants require an extraordinary degree of parental care. Lack of support from fathers and/or other family member will increase the costs borne by mothers, whereas infant health problems will reduce the evolutionary benefits to be gained (Hagen 1999). If ancestral mothers did not receive enough support from fathers or other family members, they may not have been able to "afford" raising the new infant without harming any existing children, or damaging their own health (nursing depletes mothers' nutritional stores, placing the health of poorly nourished women in jeopardy).

For mothers suffering inadequate social support or other costly and stressful circumstances, negative emotions directed towards a new infant could serve an important evolved function by causing the mother to reduce her investment in the infant, thereby reducing her costs. Numerous studies support the correlation between postpartum depression and lack of social support or other childcare stressors (Beck 2001; Hagen 1999).

Mothers with postpartum depression can unconsciously exhibit fewer positive emotions and more negative emotions toward their children, are less responsive and less sensitive to infant cues, less emotionally available, have a less successful maternal role attainment, and have infants that are less securely attached; and in more extreme cases, some women may have thoughts of harming their children (Beck 1995, 1996b; Cohn et al. 1990, 1991; Field et al. 1985; Fowles 1996; Hoffman and Drotar 1991; Jennings et al. 1999; Murray 1991; Murray and Cooper 1996). In other words, most mothers with PPD are suffering some kind of cost, like inadequate social support, and consequently are mothering less.

On this view, mothers with postpartum depression (PPD) do not have a mental illness, but instead need more social support, more resources, etc; with treatment focusing on helping mothers get what they need. (See Hagen 1999 and Hagen and Barrett, n.d.).

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Papers
Trivers, R.L. (1972) Parental investment and sexual selection. In B. Campbell (Ed.), Sexual Selection and the Descent of Man (pp. 136-179). London: Heinemann.
 * Hagen E.H. (2002) Depression as bargaining: the case postpartum. Evolution and Human Behavior. 23:323–336.

Papers

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