The model of behavioral investment as determined by mood that I previously presented has an interesting implication for the way parents treat their children. That is, depressed parents are more likely to succumb to favoritism. Specifically, as mood decreases, parental investment shifts toward children deemed more reproductively fit(1).
Parental investment involves the amount of time, energy, and other resources offered each child by their parents(2), and reproductive fitness(3) can be revealed in signals related to physical attractiveness(cuteness, etc), physical fitness, intelligence, emotional robustness, susceptibility to illness, ability to make friends, and even resemblance to one or both parents, among other cues. There is even evidence that birth weight is moderated by a mother's stress levels(lower mood), with higher stress associated with lower weights.
This is part of a larger phenomenon(first link above) in which depressed parents often unconsciously develop high quantity, low parental investment reproductive strategies in environments peceived as hostile to higher per-child investment strategies.
1. D. Beaulieu, D. Bugental (July 2008) Contingent parental investment: an evolutionary framework for understanding early interaction between mothers and children.
Evolution and Human Behavior, Volume 29, Issue 4, Pages 249-255
2. Trivers, R.L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man, 1871-1971 (pp. 136-179). Chicago, IL: Aldine.
3. Hamilton, W.D. 1964. The genetical evolution of social behavior. Journal of Theoretical Biology 7:1-52