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The Evolution of the Social Contract by Brian Skymrs

A critical book review

Seminararbeit 2011 12 Seiten

Philosophie - Philosophie des 20. Jahrhunderts


Table of Contents

1. Introduction

2. Skyrms approach to the evolution of the social contract

3. A disposition for fair division
3.1 Conception of justice
3.2 The hidden nature’s principles behind sex ratio balance
3.3 From sex ratio to justice

4. Conclusion and critique



1. Introduction

“As man advances in civilization, and small tribes are united into larger communities, the simplest reason would tell each individual he ought to extent his social instincts and sympathies to all the members of the same nation, though personally unkown to him… This virtue, one of the noblest with which man is endowed, seems to arise incidentally from our sympathies becoming more tender and more widely diffused, until they are extended to all sentient beings. As soon as this virtue is honored and practiced by some few men, it spreads through instruction and example to the young…”[1]

Darwin, in Descent of Man and Selection in Relation to Sex (1871)

Although this quote is never mentioned in Skyrms’ book The Evolution of Social Contract, it becomes clear that Skyrms believes that humans have behavioral dispositions for social behavior observable in social practices and conventions – social contracts. He sets himself the goal to trace the evolution of the social contract by applying evolutionary rational choice theory.

2. Skyrms approach to the evolution of the social contract

There are two traditions of approaching the social contract in terms of rational decision. A normative tradition that takes the position of “rational choice theory that asks what sort of contract rational choice decision takers would agree to in a preexisting ‘state of nature’”[2] and a descriptive tradition that asks why did, or how could, existing implicit social contracts have evolved and how may they continue to evolve. Skyrms takes the latter approach and regards the exploration of the question of ‘how’ and ‘why’ explicitly as non-contradictory. Instead evolutionary dynamics can provide for answers to both questions as they show a developing universal ‘sense of justice’ in human beings.[3]

Skyrms traces the evolution of the social contract with a “simple dynamical model of differential replication .. called the replicator dynamics.”[4] His theses is that social dispositions evolved over time by cultural and biological evolution which are in turn driven by differential replication under the pressure of natural selection. More explicitly, that means that from a game theoretic perspective, forms of cooperative behavior are passed on as superior strategies when it comes to the distribution of resources and survival (of the individual and the group or species).[5] Skyrms therefor avails himself of mutation and recombination as concepts of both biological and cultural evolution. Whereas a change of a strategy effected by itself is called mutation, a change of a strategy effected externally is called recombination. Culturally, “…mutation corresponds to spontaneous trial of new behaviors. Recombination of complex thoughts and strategies is a source of novelty in culture.”[6]

Furthermore, positive correlation plays a key role in Skyrms approach, stating that “if evolutionary game theory is generalized to allow for correlation of encounters between players and like-minded players, then strongly dominated strategies [that means strategies that are inferior to others in the distribution of resources and survival] … can take over the population.”[7]

3. A disposition for fair division

3.1 Conception of justice

Skyrms starts off with a commonly seen phenomena. Everyone will claim half in a symmetric situation where resources are to be divided as in the Divide the Cake[8] game.[9] Skyrms presumes that there is a mutually shared disposition to do so and sets himself the goal to explain it by applying evolutionary game theory.

The normative approach fails to provide for an answer by construing a just rule on the grounds of Nash equilibrium, according to Skyrms. “Equilibrium in informed self-interest, even when strictly construed, does not explain our conception of justice.”[10] In a Nash equilibrium one can not gain by unilaterally deviating from it. In fact, one would definitely do worse. Either he gets nothing at all as the amount exceeds 100 percent or less than the initial piece when unilaterally lowering the bid. However, not only half-half but any pair of positive claims that equal 100 percent in total is a strict Nash equilibrium. Thus even in an unequal distribution situation it is rational for both players to stick with their bid (even if it is less than half) than to get nothing at all.[11] Yet our conception of justice tells us different. Only half-half division seems to be fair. So the question remains unanswered.

3.2 The hidden nature’s principles behind sex ratio balance

To explore the human’s conception of justice Skyrms avails himselfs of the natural phenomenon: sex ratio. It has been proven that in all mammal species the number of females and males at birth is approximately equal with a slight excess of males.[12] If random mating and an equally distributed and inherited tendency to produce both sexes in equal numbers or one sex in excess is assumed, imbalances in the population will produce counter-pressures. For instance, at a preponderance of males, females would on average contribute more genes to the next generation. An individual with an inherited female siring tendency would then have a higher expected number of grandchildren until the sexes are in balance again. So in game theoretic thinking the gender siring tendency can be regarded as a strategy and the number of offsprings as a value in terms of reproductive fitness. To conclude, the tendency to produce both sexes in equal numbers is an equilibrium. The slight excess of males is explained by Skyrms by a difference in parental investment depending on the gender. Due to higher mortality before reaching maturity the expected parental expenditure for males is lower than for females.[13] In such a situation Skyrms asserts that “the evolutionary feedback leads to a propensity for equal parental investment in both sexes, rather than to equal proportions of the sexes.”[14]

Some of Skyrms assumptions seem quite questionable to me at this point. In the equal parental investment assumption Skyrms does not take into account that an individual’s reproduction time is limited. If the number of offsprings is regarded as the amount of payoff, then the offspring’s life expectancy outweighs its expected parental expenditure in a payoff consideration. Random mating on the other hand seems quite reasonable. Congenital defects may be the nature’s instrument to enforce it. Unfortunately Skyrms misses out on dealing with empirical studies of sex ratio developments. For instance, he could have taken a look into the statistics of sex ratio at birth during the colonization of Australia. The sex ratio imbalance in Australia has only began to be eased with assisted immigration from 1832 on but remained existent for decades[15] - enough generations of male preponderance worth scientific consideration to validate the Skyrms thesis of counter-pressures at sex ratio imbalances.

3.3 From sex ratio to justice

Skyrms then draws parallels of evolutionary theory to rational choice theory explaining the evolution of the social contract – such as fair division in the bargaining game - by applying the described nature’s mechanisms that keep sex ratios in balance to distributive choice situations. In the bargaining game described as Divide the Cake the proportion of the resource represents the fitness of a strategy (or more precisely the fitness of an individual or a proportion of the population playing that strategy). Skyrms finds out, that if iteration is applied to the game, demand-half is the only evolutionary stable equilibrium. Yet that does not mean that everyone necessarily plays the strategy of fair division and perfect justice must evolve. There are stable mixed states of the population, where different proportions of the population play different strategies. In the long run the population will spent a long amount of time in the fair division equilibrium relative to other possible equilibria. If there is some tendency for like to interact with like then attraction to fair division is even stronger.


[1] Darwin, C. (1871), pp. 102

[2] Skyrms, B. (1996), pp. ix

[3] Cf. Skyrms, B. (1996), pp. ix and cf. Sober, E. (2005), pp. 5

[4] Skyrms, B. (1996), pp. ix

[5] Cf. Barret, M. et al (1999), p. 237 and cf. Sober, E. (2005), pp. 9-11

[6] Skyrms, B. (1996), pp. x

[7] Skyrms, B. (1996), pp. x

[8] see Appendix

[9] Skyrms, B. (1996), pp. 3-5

[10] Skyrms, B. (1996), pp. 5

[11] Cf. Skyrms, B. (1996), pp. 4-7

[12] Cf. Skyrms, B. (1996), pp. 1-3

[13] Cf. Skyrms, B. (1996), pp. 7-9

[14] Skyrms (1996), pp. 9

[15] Jupp, J. (Editor) (2001), pp. 30, 31, 41


ISBN (eBook)
ISBN (Buch)
524 KB
Institution / Hochschule
Universität Bayreuth
evolution social contract brian skymrs




Titel: The Evolution of the Social Contract by Brian Skymrs