endeavor—they will need to stop investing in the offspring. The ability to defect from costly
endeavors is a key component of any cooperative enterprise. Non-kinship based reciprocity
cannot evolve or be maintained in the population if individuals do not have the ability to defect
(e.g., Axelrod & Dion, 1988; Axelrod & Hamilton, 1984), a result with the disturbing
implication that child neglect and abandonment are necessary for the evolution and maintenance
of cooperative childrearing.
What if an individual cannot easily defect? Evolutionary models suggest that cooperative
contracts can be enforced by imposing costs on those who defect. In the US and many other
countries, women face extremely high social costs for defecting from their offspring, for
example. Due to the social costs, a mother cannot defect from child rearing even if she has no
social support or the infant has problems, that is, even if her benefits are significantly
outweighed by her costs. Women who do defect from child rearing may face serious criminal
charges.
In ancestral environments mothers may also have faced high social costs for defecting. Both
the father and other family members had a fitness interest in the offspring, and, rather than
investing themselves, they may have attempted to coerce the mother into providing the child
care by threatening her with social costs should she neglect the offspring. The mother was then
trapped between a rock and a hard place: she couldn’t afford to invest in the offspring, but she
couldn’t afford to defect either. This was an extraordinarily costly situation for the mother, and
extraordinary efforts to negotiate greater investment, or find some way to minimize the costs of
defecting are called for.
What power does the mother have to negotiate greater investment? I argue that major
depression and its attendant symptoms like the loss of interest in most or all activities, significant
weight loss, hypersomnia, psychomotor retardation, fatigue and loss of energy, and a diminished
ability to think or concentrate, have at least three negotiating functions. The first function is
analogous to a worker who threatens to quit or go on strike in an effort to negotiate a larger
salary. Mothers, as key investors in new, existing, and future offspring, are a valuable source of
benefits and can negotiate greater investment by putting these benefits at risk. By losing interest
in herself, a depressed mother is making a very credible threat of defecting from cooperative
endeavors that others find reproductively beneficial. She is not providing the benefits to others
that she has previously agreed to provide, and is putting her ability to provide such benefits at
risk. By holding important benefits hostage (e.g., the life of the newborn, investment in existing
offspring, and her ability to produce future offspring) the mother may be able to elicit greater
investment from others. The threat of defection is credible and robust to bluff-calling since the
mother’s costs are currently outweighing her benefits.
Second, if mothers fail to negotiate greater investment they may indeed suffer social costs
for defecting, but the imposition of social costs is not free. Those who impose social costs (e.g.,
the father and family members) will have to decide whether to continue to pay the costs of
coercion when they aren’t generating much return—the depressed mother is clearly willing to
not care for herself rather than continue to invest in the offspring. The father and family
members may decide that it is not worth it to continue to attempt to coerce the mother and let her
defect. This outcome is probably rare since mothers are likely to be successful in negotiating
increased investment and thus do not ultimately abandon or neglect their offspring.
Finally, because depression prevents the mother from pursuing other profitable opportunities,
it may mitigate the social costs of defecting. Those who defect from social contracts—those
who cheat—are often punished. Receiving a benefit without providing a benefit in return is
perceived by most people as a very reliable cue of cheating (e.g., Cosmides & Tooby, 1992).
Mothers with major depression are not providing benefits, but they are not receiving or pursuing
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