Imagine a population of competing organisms. Some are aggressive (fight often, take risks), some are passive (avoid fights, conserve energy). Aggression is costly—fights produce injuries—but it pays off if you win. If the population is mostly passive, aggression is highly successful (you win every fight and get all the resources). But if the population is mostly aggressive, aggression is costly and often loses (you spend energy fighting and still lose).
This is the core of frequency-dependent selection: the success of a strategy depends on its frequency in the population.1
When aggression is rare, each aggressive individual does very well (rare + effective = high payoff). This causes more individuals to adopt aggression. But as aggression becomes common, the payoff drops (now most fights are against other aggressive individuals, and you're just burning energy). At some point, the payoff to aggression drops below the payoff to passivity. Individuals switch to passivity. But now passivity becomes common, which means aggression is rare again, which means aggression's payoff climbs. The strategies oscillate around an equilibrium point.2
At equilibrium, the expected payoff to each strategy is identical. This seems paradoxical: if each strategy is equally successful, why would anyone adopt one over the other? The answer is that at equilibrium, they're equally successful only on average. Individual outcomes are still determined; both strategies remain present in the population because neither has a consistent edge.3
This equilibrium is evolutionarily stable: if the frequency of strategies deviates from equilibrium (more aggression than equilibrium calls for), the payoff to aggression drops below passivity, causing individuals to shift toward passivity, pushing frequency back toward equilibrium. If frequency deviates the other way (more passivity), aggression's payoff climbs, causing shift toward aggression. The system naturally returns to equilibrium.4
Frequency-dependent selection explains why human populations maintain persistent behavioral variation in domains where you might expect evolution to favor a single optimal strategy.
Mating Strategies: Some males pursue monogamous pair-bonding (invest heavily in one female and her offspring). Others pursue promiscuous strategies (invest minimally and mate with many females). Each strategy's success depends on frequency. If most males are investing, promiscuous strategy pays off (mate with investing males' partners, get free rider benefits). If most males are promiscuous, female preference for investing males increases, making monogamous strategy more successful. The population maintains both strategies in stable equilibrium.5
Cooperation Levels: Some individuals are highly cooperative (reciprocal, trusting). Others are defectors (exploit cooperation without reciprocating). Cooperation works well when cooperators are common (you're surrounded by people who reciprocate). Defection works well when cooperators are common (you exploit them without consequence). As defection becomes common, cooperation's payoff drops, but defection also drops (fewer people to exploit). Equilibrium maintains both strategies.6
Cheating and Cheater-Detection: Some individuals invest heavily in honesty (reliable, trustworthy). Others are skilled deceivers. Honesty is successful when people are trusting (you benefit from their credulity without the cost of deception). Deception is successful when people are highly trusting (low detection risk). As deception becomes common, people get more suspicious, which increases detection and punishment of deceivers, which reduces deception's payoff. Honesty becomes relatively more successful again.7
Status Competition Intensity: Some individuals compete intensely for status (aggressive, display-oriented, risk-taking). Others accept lower status (conservative, modest, risk-averse). Intense competition is successful when high status is valuable and achievable. As competition intensifies, status becomes harder to achieve (more competitors), and the cost of losing increases. Conservative strategy becomes more successful. At equilibrium, the population maintains both types.8
The existence of behavioral polymorphism puzzles evolutionary thinking: if evolution favors the most successful strategy, shouldn't it eliminate inferior strategies? The frequency-dependent selection answer is: no, because strategies are successful relative to their frequency. At equilibrium, no single strategy is superior; they're all equally successful on average.9
This explains human behavioral diversity better than assuming one "optimal" strategy that everyone should adopt. Humans maintain variation in cooperation, honesty, mating strategy, and status-seeking because this variation is stable. The population oscillates around equilibrium frequencies where each strategy is equally successful.10
Yet there's an important caveat: frequency-dependent selection produces stable equilibrium only when populations are well-mixed (everyone interacts with everyone). In structured populations (groups, networks, local communities), different equilibria can emerge. A group can tip toward high cooperation or toward exploitation depending on initial conditions.11
Modern psychology often treats individual differences (personality, behavioral style, strategy) as deviation from a norm—assuming that some strategies are "better" and individuals differ in how well they achieve the ideal. But frequency-dependent selection suggests that individual differences are equilibrium features of populations.
A person who is highly competitive isn't deviating from an ideal of cooperation; they're occupying an equilibrium position in a competition-cooperation space. A person who is highly trusting isn't naïve compared to a suspicious person; they're occupying an equilibrium position in a trust-suspicion space. Both are equally successful on average; their individual outcomes depend on whom they interact with.12
Evolutionary Stability Theory (ESS) vs. Empirical Variation
Evolutionary stability theory (developed by Maynard Smith) predicts that evolution should produce evolutionarily stable strategies—strategies that, once established, resist invasion by alternative strategies. ESS logic seems to predict that populations should converge on a single optimal strategy.13
Yet empirical observation shows persistent polymorphism: multiple behavioral strategies persist in populations. How can this be stable if ESS predicts convergence?
The answer is frequency-dependent selection: the stable state can involve multiple strategies if each is equally successful at equilibrium frequency. This is a frequency-dependent ESS rather than a pure-strategy ESS. The population is stable with multiple types maintained at equilibrium frequencies.14
Wright vs. Adaptationist Dogma on Behavioral Diversity
Adaptationists often assume that observed behavior is optimal—that humans have evolved to be highly cooperative (among cooperators), or highly competitive (where competition is adaptive), or highly trustworthy (where reputation matters). Individual differences are treated as noise around the optimal strategy.15
Wright suggests that individual differences are not noise; they're features of equilibrium populations. A population needs both highly cooperative and moderately selfish individuals to maintain stable reciprocal altruism. A population needs both trusting and suspicious individuals to maintain honest exchange. The diversity is the adapta
tion, not the problem.16
Emotional responses are calibrated to strategies and their frequency. A person who is suspicious of others' trustworthiness is adopting (consciously or not) a frequency-dependent strategy: suspicion is adaptive when deception is common, counterproductive when deception is rare.17
The handshake is that psychological variation in trust, competitiveness, and cooperativeness can be understood as frequency-dependent equilibrium in emotional systems. The variation isn't personality pathology; it's stable maintenance of behavioral diversity that equilibrates at population-level success.18
Different institutions select for different frequencies of behavioral types. A highly competitive institutional environment selects for intense competitors; a cooperative institutional environment selects for cooperators. Institutional change shifts the equilibrium frequencies.19
The handshake is that historical changes in institutions (shift from small-group reciprocity to market exchange, from face-to-face societies to anonymous large organizations) change the equilibrium frequencies of behavioral strategies. What was successful in ancestral environments might be unsuccessful in modern institutions, not because the strategy changed, but because the equilibrium frequency shifted.20
If behavioral strategies exist in frequency-dependent equilibrium, then there is no single "optimal" human personality or psychology. The person who is highly competitive, the person who is highly cooperative, the person who is suspicious, the person who trusts freely—each occupies an equilibrium position where they're equally successful (on average) as others in their population. This means your personality isn't a measure of how well you've achieved an ideal; it's a stable position in a frequency-dependent equilibrium.21
Yet this has a troubling implication: if you're competing in a domain where your strategy is losing frequency (your local equilibrium has shifted), you might experience yourself as failing without anything actually being wrong with you. A cooperative person in a competitive environment might experience constant failure because cooperation has become less advantageous relative to competition. The problem isn't the strategy; it's the environment.22