Cooperation in Humans: Kin Selection, Altruism, and Social Contract Theory

Basics of Kinship and Inclusive Fitness

The ultimate fate of a heritable trait is determined by the impact it has on the fitness of the individuals who carry it. Carriers of a gene include relatives other than just one's own offspring. In this biological framework, all relatives are viewed as "vehicles of fitness," although they differ significantly in their reproductive value. Selection favors the evolution of psychological and physiological mechanisms designed to help close kin more than distant kin or strangers. This logic is derived from the Theory of Inclusive Fitness and Hamilton’s Rule.

Theory of Inclusive Fitness

Proposed by W.D. Hamilton (1964), this theory represents a fundamental reformulation of the theory of natural selection. Hamilton argued that the classical concept of personal reproductive success (also known as personal fitness, classical fitness, or Darwinian fitness) is too narrow to fully explain natural selection.

Instead, selection favors traits that cause an organism’s genes to be passed on to the next generation, regardless of whether the organism produces offspring directly or indirectly by helping kin. Inclusive fitness is defined as the sum of:

Direct Individual Reproductive Success: The individual's own fitness through direct descendants.
Indirect Effects: The impact of that individual’s actions on the reproductive success of genetic relatives, weighted by the degree of relatedness ( $r$ ).

Hamilton’s Rule and Relatedness Coefficients

Hamilton’s Rule suggests that organisms can increase their fitness not only by producing their own offspring but also by helping genetic relatives to survive and reproduce. Parental care is considered a special case of care for those with whom one shares genes by common descent. Hamilton's seminal works include The Genetical Evolution of Social Behaviour I and II.

The degree of genetic relatedness ( $r$ ) determines the strength of the selection for altruistic behavior:

Yourself: $r = 1$
Identical Twin: $r = 1$
Parent or Child: $r = 1/2$
Sibling (Full Sister or Brother): $r = 1/2$
Grandparent or Grandchild: $r = 1/4$
Half-Sibling: $r = 1/4$
Aunt, Uncle, Niece, or Nephew: $r = 1/4$
Great-Grandparent or Great-Grandchild: $r = 1/8$
Great-Aunt, Great-Uncle, Great-Niece, or Great-Nephew: $r = 1/8$
First-Cousin: $r = 1/8$
First-Cousin-Once-Removed: $r = 1/16$
Second-Cousin: $r = 1/32$
Stranger: $r = 0$

General formulas for relatedness:

$n$ th level ancestor or descendant: $1/2^n$
$n$ th cousin: $1/2^{2n+1}$
$n$ th cousin, $m$ times removed: $1/2^{2n+m+1}$

Universal Aspects of Kinship

Daly, Salmon, and Wilson (1997) identified several universal features of human kinship systems:

Cooperation and Solidarity: Helping behavior is organized on the basis of relatedness.
Encouragement of Altruism: Elders often encourage younger members to help "collateral" kin.
Knowledge of Blood Relatives: Individuals possess specific knowledge of who their real (biological) relatives are.
Kinship Terminology: Kinship terms (e.g., "brother") are sometimes used to influence the behavior of non-kin.
Ego-Centered Terminology: Terms are usually relative to the speaker (e.g., "my mother," "my sister").
Sex and Generation Distinctions: All systems make distinctions based on sex (e.g., grandfather vs. grandmother) and generation.
Organization by Closeness: Relationships are organized by biological distance ( $r$ ).
Self-Concept: An individual's identity is often based on their position within the kin network (e.g., being a daughter, a brother).

Empirical Tests of Kin Selection

Patterns of Helping (Essock-Vitale & McGuire, 1985)

In a study of 300 women in Los Angeles involving over 5,000 descriptions of giving and receiving help, results showed:

Significantly more acts of helping were directed toward close kin than toward distant kin or non-kin.
Helping increased as a function of the recipient’s reproductive value, meaning help generally flowed from older individuals to younger individuals.

Survival in Maritime Disasters: The RMS Titanic

The sinking of the Titanic on April 15, 1912, provided a case study for survival determinants. Statistics for the 2,224 passengers and crew include:

Survivors: 710 (31.9%)
Deaths: 1,514 (68.1%)
Gender and Social Norms: Women were far more likely to survive than men (75% vs. 20%). Women in couples were even more likely to survive, whereas men in couples were less likely to survive, suggesting specific social and kinship dynamics under pressure.

Wealth Inheritance (Smith et al., 1987)

An analysis of 1,000 probated wills in Vancouver, British Columbia (552 men, 448 women) showed patterns of resource distribution based on relatedness:

Kin (excluding spouses): Received 55.3% of bequests.
Spouses: Received 36.9%.
Non-kin: Received only 7.7%.
$r = 0.5$ Kin (Offspring): Received 38.6% of the total, with sons (19.2%) and daughters (19.4%) receiving nearly equal amounts.
$r = 0.5$ Kin (Siblings): Received 7.9% (Brothers 3.2%, Sisters 4.8%).
$r = 0.25$ Kin (Nieces/Nephews/Grandchildren): Received 8.3%.
$r = 0.125$ Kin (Cousins): Received 0.6%.

Sex Differences in Wills (Judge & Hrdy, 1992):

Women tend to distribute their estates more widely than men.
Men often leave estates to wives with the instruction to pass them to children.
Women rarely leave such trusts to husbands; when they do mention husbands, it is often with a qualifier like "only if he remains unmarried."

Investment by Grandparents

Evolutionary theory predicts variability in grandparental investment due to paternity uncertainty:

Maternal Grandmother: Maximum certainty ( $r = 0.25$ ). She knows the child is her daughter's and knows her daughter is the mother. She usually invests the most.
Paternal Grandfather: Minimum certainty. He is unsure if he is the genetic father of his son and unsure if his son is the genetic father of the grandchild. He usually invests the least.

Importance of Kin (Salmon & Daly, 1996)

In a study of 24 adult sibling pairs, sisters invariably recalled more relatives in their genealogies than brothers did.
In a study where 300 subjects provided 10 "who am I?" descriptions, females were significantly more likely than males to mention their family roles.

Social Organization and Reciprocal Altruism

Benefits of Social Organization

Reduction in predator pressure (improved detection or repulsion).
Improved foraging efficiency (hunting or finding spread-out resources).
Improved defense of limited resources against other groups.
Improved care of offspring (communal feeding and protection).

Reciprocal Altruism and Social Contract Theory

Reciprocal altruism involves cooperation between non-relatives where both parties benefit from a "Gain in Trade" over time (e.g., Sharing food when one is successful and receiving food when unsuccessful). This requires trust and several cognitive abilities as outlined in Social Contract Theory (Cosmides & Tooby, 1992):

Ability to recognize many different individual humans.
Ability to remember interactions with these individuals.
Ability to communicate values and model the values of others.
Ability to represent costs and benefits regardless of the items being exchanged.

The Prisoner’s Dilemma

This game models the conflict between cooperation and defection:

Reward for Mutual Cooperation ( $R=3$ ): Both stay silent.
Punishment for Mutual Defection ( $P=1$ ): Both confess.
Temptation to Defect ( $T=5$ ): One confesses/implicates the other; the defector goes free with a reward.
Sucker’s Payoff ( $S=0$ ): One stays silent while the other implicates them; the silent one gets a stiff sentence.

Biological Examples and Non-Kin Altruism

Vampire Bats: Wilkinson (1984) and Carter & Wilkinson (2013) demonstrated that reciprocal food sharing in vampire bats is predicted more by reciprocal help than by relatedness.
Non-Kin Altruism: Altruism can extend across species, such as an Australian firefighter giving water to a Koala survivor of a bushfire or Koalas seeking water from humans during heatwaves.