Inclusive Fitness and Its Implications The concept of "inclusive fitness." - Distinction between direct and indirect fitness. - Degree of relatedness. Implications for the evolution of human capacity for altruism. - Hamilton's rule. Implications for altruistic behavior in the here and now. - Psychological research on altruistic decision-making.

Implications for the evolution of altruism. (“kin selection”) Reading for today… Dawkins, Chapter 6 The concept of inclusive fitness. Implications for the evolution of altruism. (“kin selection”) Kinship and kin recognition.

Inclusive fitness = direct fitness + indirect fitness The concept of “inclusive fitness” From point of view of genes, “vehicles" should be designed to do things that make copies of those genes…   1. Behaviors that enhance vehicle's own reproductive success (“direct fitness”).   2. Behaviors that enhance reproductive success of other vehicles carrying those same genes (“indirect fitness”). Inclusive fitness = direct fitness + indirect fitness

(genetic relatedness) The concept of “inclusive fitness” Genes that promote inclusive fitness (either directly or indirectly) are more likely to become common within a population.   Kinship (genetic relatedness) really matters!

Degree of relatedness (numerical index of kinship): Probability that 2 people with have a gene in common, above and beyond the average number of genes that all humans share.

Implications for the evolution of human capacity for altruism: (“kin selection”) Genes promoting altruistic behavior will spread within population if direct fitness costs associated with altruism are outweighed by the indirect fitness benefits of altruism. William Hamilton

Implications for the evolution of human capacity for altruism: (“kin selection”) Direct fitness costs associated with altruism = Extent to which own altruistic behavior reduces own reproductive success   Indirect fitness benefits of altruism = increases other's reproductive success X Degree of relatedness between self and other (c) (b) (r)

Implications for the evolution of human capacity for altruism: (“kin selection”) Genes promoting kin-directed altruism will spread within a population if r × b > c. (“Hamilton's rule”)

Implications for altruistic behavior in the here and now.   Broad implication: If capacity for kin-based altruism did evolve within human populations, then our psychological mechanisms may have evolved in such a way as to be sensitive to the implications of behavior for both direct and indirect fitness. One key variable with implications for direct fitness: c (Cost, in terms of own reproductive success)   Two key variables with implications for indirect fitness: b (Benefit, in terms of other's reproductive success) r (Degree of relatedness between self and other)

Implications for altruistic behavior in the here and now. Specific predictions: - When kinship is greater, people will be more likely to help. - This effect will occur most strongly in life-or-death situations (compared to more “everyday” helping situations).

[ Crude mathematical illustration ] b c (r × b) - c   1 10 5 .5 .25 -2.5 .125 -3.75 [ Crude mathematical illustration ] r b c (r × b) - c   1 100 50 .5 .25 -25 .125 -37.5

[ Crude mathematical illustration ]

Psychological research on altruistic decision-making: (Burnstein, Crandall, & Kitayama, 1994)

(Burnstein, Crandall, & Kitayama, 1994)

Implications for altruistic behavior in the here and now. Some additional specific predictions for kin-directed altruism: - When a genetic relative’s reproductive potential is greater, people will be more likely to help them. - This effect will occur primarily in life-or-death situations (compared to more “everyday” helping situations). What characteristics of a person might provide clues to their reproductive potential?

(Burnstein, Crandall, & Kitayama, 1994)

(Burnstein, Crandall, & Kitayama, 1994)