Kinship and Inclusive Fitness   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.

Dawkins, Chapter 6: The concept of inclusive fitness. Implications for the evolution of altruism (“kin selection”) Kinship and kin recognition. Dawkins, Chapter 8: The concept of parental investment (and some of its implications)

From point of view of a gene: Organisms are “vehicles"   Organisms are “vehicles"

From point of view of a gene:   “Vehicles" should be designed so that they do things that... 1. Maximize vehicle's own reproductive success. (Direct implications for reproductive fitness) 2. Maximize reproductive success of other vehicles carrying that same gene. (Indirect implications for reproductive fitness)

Inclusive fitness: Inclusive fitness = direct fitness + indirect fitness Genes that promote inclusive fitness (either directly or indirectly) are more likely to become common within a population. Because of this: 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”) William Hamilton

Direct fitness costs associated with altruism = Extent to which own altruistic behavior reduces own reproductive success (c)   Indirect fitness benefits of altruism = increases other's reproductive success (b) X Degree of relatedness between self and other (r)

Evolution of kin altruism: If r × b > c, then gene promoting kin altruism will spread within population (“Hamilton's rule”)  

Implications for altruistic behavior in the here and now.   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. 1 key variable with implications for direct fitness: c (Cost, in terms of own reproductive success) 2 key variables with implications for indirect fitness: b (Benefit, in terms of other's reproductive success) r (Degree of relatedness between self and other)

Some specific implications: - When r (kinship) is greater, people are more likely to help. - This effect of r matters primarily in life-and-death situations (compared to more “everyday” kinds of helping situations).  

r b c r × b - c   1 10 5 .5 .25 -2.5 .125 -3.75 r b c r × b - c   1 100 50 .5 .25 -25 .125 -37.5

Do helping decisions actually follow these principles? Yes. (Burnstein, Crandall, & Kitayama, 1994)

(Burnstein, Crandall, & Kitayama, 1994)

More implications: - When b (reproductive benefit) is greater, people are more likely to help. - This effect of b matters primarily in life-and-death situations.

What features of a person might serve as evidence of b? (What features provide clues to their future reproductive potential?)

(Burnstein, Crandall, & Kitayama, 1994)

(Burnstein, Crandall, & Kitayama, 1994)