Evolutionary Perspectives on Prosocial Behavior

Jeffry A. Simpson

University of Minnesota

Overview of the Talk

•  Describe the environments in which early humans evolved

•  Briefly review major evolutionary theories, which reveal the theoretical “expansion” of how and why prosocial actions could have evolved in humans

•  Discuss findings that support this expansion

Features of Ancestral Environments

•  For 98% of human evolutionary history, our ancestors: –  lived as hunters and gatherers (Kelly, 1995), with men doing most hunting and

women doing most gathering (Wood & Eagly, 2002) –  dwelt in small, cooperative bands (Richerson & Boyd, 2005) –  were biologically-related to most other tribe/group members (Foley, 1992) –  met strangers infrequently (Wright, 1994) –  lived in the same group their entire lives (Kelly, 1995) –  were raised by kin, extended family members, and tribe members, with older

sibs helping to raise younger sibs (Eibl-Eibesfeldt, 1989) –  cooperated extensively with kin and non-kin (Brewer & Caporeal, 2006)

•  These conditions are ideal for the evolution of altruism (Cosmides & Tooby, 1992).

•  Cooperative groups were the primary “survival strategy” of early humans (Brewer & Caporeal, 1990).

Natural Selection

Natural Selection Theory

(Darwin, 1859, 1871) •  Focused on “survival of the fittest” and differential

reproduction at the level of the individual. •  The theory explains why organisms act on their own

self-interests in many situations. •  Although Darwin speculated about stronger forms of

prosociality in humans, his original theory did not explain how or why it could have evolved.

Inclusive Fitness

Natural Selection

Inclusive Fitness Theory

(Hamilton, 1964) •  Shifted attention to “kin selection” and genes as the unit

of selection. •  Why do some individuals not reproduce, but help their

relatives raise their children? •  This behavior should have been selected when the costs

of not reproducing to help kin raise their children were less than the benefits of doing so, given the degree of biological relatedness to the relative’s child.

Reciprocal Altruism

Inclusive Fitness

Natural Selection

Reciprocal Altruism Theory

(Trivers, 1971) •  Focused on how selective altruism could have evolved

between non-kin. •  There should have been situations when non-kin who

developed beneficial exchange relationships could have facilitated each other’s survival and reproductive success.

•  Reciprocal altruism: Should have evolved in species that: 1. have longer life spans

2. repeatedly interact with the same individuals 3. are mutually dependent 4. have weaker dominance hierarchies 5. can benefit from other ingroup members during

conflicts with outgroups 6. invest heavily in offspring and parental care

Group Selection

Reciprocal Altruism

Inclusive Fitness

Natural Selection

Group Selection Theory

(Sloan Wilson & Sober, 1994) •  In certain situations, groups could have been the unit of

selection. •  If certain groups were more productive, skillful, or inventive

than others, individuals in “successful” groups should have left more descendants.

•  Because cohesion and organized division of labor are critical to effective group functioning, strong prosocial tendencies toward all ingroup members could have been selected.

•  However, the ratio of costs-to-benefits of remaining in a group must be very low and the % of altruists in a group must be very high to start and sustain group selection.

Gene-Culture Co-evolution

Group Selection

Reciprocal Altruism

Inclusive Fitness

Natural Selection

Gene-Culture Co-Evolutionary Theories

(e.g., Richerson & Boyd, 2005)

•  Group selection may have occurred through genetic and cultural evolution.

•  During evolutionary history, rapid environmental and climate changes may have led to special social learning and imitation capacities in humans.

•  These capacities allowed new knowledge, information, and innovations to be transmitted more efficiently and reliably within and across generations in certain groups.

Gene-Culture Co-Evolutionary Theories

•  Cultural practices unique to a group (e.g., discovering

better ways to grow crops, hunt animals, or defend territories) could have enhanced the survival and reproduction of members of that group.

•  Large between-group differences in cultural practices could have been maintained by: 1. Strong moralistic punishment of cheaters or defectors

in groups. 2. Strong pressure to conform to group rules and

norms. 3. Sustained conflict with outgroups.

Evidence Supporting Gene-Culture

Co-Evolutionary Theories

•  Purely gene-centered theories cannot explain the extent and depth of human prosocial tendencies, especially toward strangers.

•  If Gene-Culture Co-Evolutionary theories are correct, people should: 1. readily conform to group rules and norms. 2. distinguish ingroup from outgroup members and act on

these distinctions. 3. punish persons who violate important group rules/norms.

Evidence Supporting Gene-Culture

Co-Evolutionary Theories

• Several studies have documented these effects: 1. People are strongly motivated to conform to

group norms and pressures (Cialdini & Goldstein, 2004), even when tasks are simple and groups are temporary (e.g., Asch, 1956; Sherif, 1936).

Evidence Supporting Gene-Culture

Co-Evolutionary Theories

2. People automatically distinguish ingroup from outgroup members and discriminate against outgroups (Brewer & Brown, 1998), both when differentiation is trivial or random (Tajfel & Turner, 1979) and when it is important (e.g., Sherif et al., 1961).

Evidence Supporting Gene-Culture

Co-Evolutionary Theories

3. People display strong reciprocity (e.g., take on the

costs of rewarding or punishing others when cooperation is essential for group cohesion or desirable outcomes), even when costs are very high and the “enforcer’s” self-interest is harmed (Fehr & Fischbacher, 2003).

Strong reciprocity ensures that cheaters and non-

cooperators do not destroy cooperation and good-will within groups.

Conclusions

•  In sum Darwin left room for all of these evolutionary

models of prosocial behavior, each of which addresses different adaptive problems.

•  Inclusive Fitness Theory: Explains extreme forms of self-sacrifice that evolved to protect individuals’ direct genetic interests, especially when biological relatives needed help in life-or-death situations.

•  Reciprocal Altruism Theory: Explains more common, less costly prosocial acts that increased individuals’ fitness, particularly when resources were limited, unpredictable, or difficult to obtain and the cooperation of others was necessary.

Conclusions

•  Gene-Environment Co-Evolutionary Theories: Explain

other forms of prosocial behavior (e.g., helping all members of one’s group, even when reciprocal alliances have not been established) that increased fitness by allowing individuals to benefit from established cultural practices and living in a highly cohesive, productive group.

•  Given the many obstacles to survival and reproduction, humans should have evolved to take advantage of multiple routes to enhancing their fitness.

Thank You