Social behavior sits at the intersection of psychology, sociology, and biology, and the MCAT tests it in that integrated way: recognize a scenario and match it to a named concept, distinguish subtly similar theories, and understand why organisms behave socially. A unifying frame: all social behavior has both proximate psychological causes and ultimate evolutionary explanations, and the MCAT expects you to reason at both levels.
Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.
Attraction
The Building Blocks of Liking
Must knowThese factors predict attraction:
- Proximity (propinquity effect) is the single strongest predictor — you bond with people you encounter repeatedly. Its mechanism is the mere exposure effect: more familiarity → more positive evaluation, even unconsciously (Zajonc). High-yield because it explains repeated advertising, jingles, and reduced prejudice with contact.
- Similarity (attitudes, values, demographics) is next strongest — opposites rarely attract lastingly. Similar others validate your worldview and are easier to interact with.
- Reciprocal liking — knowing someone likes you increases your liking of them.
- Physical attractiveness strongly predicts initial attraction. The halo effect amplifies it: attractive people are also rated more intelligent, warm, and competent. The matching hypothesis adds that partners tend to be of roughly equivalent attractiveness.
complementarity (attraction to those whose traits complement yours) is a real but weaker, more context-specific effect than similarity.
From Liking to Love
Must knowSternberg's triangular theory of love: love = intimacy (closeness) + passion (arousal) + commitment (decision to maintain). Combinations yield different loves; the high-yield distinction is passionate love (intense, arousal-driven, early) vs. companionate love (enduring affection and commitment as passion fades). Consummate love = all three.
Two frameworks for how relationships persist:
- Social exchange theory — relationships as cost-benefit calculations; people stay when rewards exceed costs and beat their comparison level (expectations) and comparison level for alternatives.
- Equity theory — satisfaction depends on fairness: both partners perceiving a roughly equal ratio of benefits to contributions. Over-benefiting breeds guilt, under-benefiting resentment.
Quick check: A student likes her lab partner more as the semester progresses, even though they disagree on most political topics. What concept best explains the increase?
Answer: The mere exposure effect — repeated contact increases positive evaluation independent of content. The disagreement makes similarity weak, but proximity/repeated exposure does the work.
Aggression
Must knowAggression is behavior intended to harm another organism. Must know the distinction: hostile aggression (harm as an end, driven by anger) vs. instrumental aggression (harm as a means to another goal, e.g., territory). They have different predictors.
Psychological Theories
Know the logicUnderstand each mechanism, not the dates:
- Frustration-aggression hypothesis (Dollard): blocking a goal produces an aggressive drive. The original "always" version was too absolute; Berkowitz's revision says frustration creates a readiness triggered by an aggressive cue (e.g., a weapon) — the weapons effect.
- Social learning theory (Bandura): aggression is learned by observing and imitating models. The Bobo doll experiment — children imitated an adult beating an inflatable clown without being reinforced — is direct evidence for observational learning. Drives media-violence passages.
- Excitation transfer theory (Zillmann): arousal from one source (exercise, a scary movie) is misattributed to a second, amplifying aggression.
Biological and Environmental Factors
Must knowThe biology: testosterone lowers the threshold for challenge/status responses (not a direct cause); low serotonin → increased impulsivity and aggression; the amygdala is central to emotional aggression; the prefrontal cortex provides the impulse-control brake.
Environment also shapes aggression: deindividuation (loss of self-awareness/accountability in groups or anonymity) releases normal restraints, and cultural norms modulate it (e.g., "cultures of honor" endorsing retaliatory violence).
Quick check: Children in a high-violence neighborhood become more aggressive over time without being directly reinforced for their own aggressive acts. Which theory best explains this?
Answer: Social learning theory (Bandura) — the children model observed aggression through vicarious learning, not direct reinforcement.
Attachment
Must knowAttachment is an enduring emotional bond, especially infant–caregiver, that shapes relationships across the lifespan. The MCAT tests the researchers, studies, and styles with precision.
Harlow's Rhesus Monkeys
Must knowHarlow gave infant monkeys a wire surrogate that provided milk and a cloth surrogate that provided contact comfort but no food. Infants overwhelmingly preferred the cloth mother, showing contact comfort matters more than feeding for attachment — refuting the drive-reduction (food = attachment) assumption. Passage-level monkeys raised in isolation showed severe, lasting abnormalities, demonstrating a sensitive period for social development.
Bowlby's Attachment Theory
Must knowBowlby framed attachment as an evolved proximity-seeking system (proximity to a protective adult aided survival). Key terms: the secure base (from which a child explores) and safe haven (to which the child returns for comfort), plus internal working models — mental representations of self and others, built from early attachment, that guide future relationships.
Ainsworth's Strange Situation and Attachment Styles
Must knowAinsworth's Strange Situation uses structured separations and reunions to classify infant attachment. Must know the four styles and their reunion behavior:
| Style | Behavior on reunion | Caregiver pattern |
|---|---|---|
| Secure | Distressed when separated; easily soothed; resumes exploring | Consistently responsive |
| Anxious/Ambivalent | Very distressed; not easily soothed; clingy yet angry | Inconsistently responsive |
| Avoidant | Minimal distress; ignores caregiver on return | Unresponsive/rejecting |
| Disorganized | Contradictory, confused behavior (often abuse contexts) | Frightening/frightened caregiver |
Secure attachment predicts better emotional regulation and adult relationships.
Quick check: A toddler shows little distress when the mother leaves and turns away when she returns. Which attachment style, and what caregiving pattern predicts it?
Answer: Avoidant, predicted by a consistently unresponsive/rejecting caregiver — the child has learned that signaling distress yields no comfort.
Altruism as Social Behavior
Must knowAltruism is behavior that benefits another at a cost to the actor; psychologically, prosocial helping motivated by concern for others. The MCAT tests both the conditions that promote/inhibit helping and the evolutionary explanations (below).
The Bystander Effect
Must knowThe bystander effect — the probability any one person helps decreases as the number of bystanders increases (Latané & Darley, prompted by the Kitty Genovese case). Two drivers:
- Diffusion of responsibility — with others present, each feels less personally responsible ("someone else will help").
- Pluralistic ignorance — in ambiguous emergencies, each bystander reads others' calm as evidence it's not an emergency.
Factors that increase helping: unambiguous emergency, personalization (eye contact), similarity to victim, good mood, low cost, rural setting (urban-overload hypothesis).
The Empathy-Altruism Hypothesis
Know the logicThe debate: Batson's empathy-altruism hypothesis holds that genuine empathy produces genuinely altruistic motivation (people help even when easy escape is available). This contrasts with egoistic models — e.g., Cialdini's negative-state relief model, where helping is really about relieving the helper's own distress.
Quick check: Mia sees someone collapse in a crowded station, looks around at uncertain bystanders, concludes the person is probably drunk, and does nothing. Which concept explains her inaction?
Answer: Pluralistic ignorance — she reads others' calm to define an ambiguous situation as a non-emergency.
Social Support
Must knowSocial support = resources (emotional, material, informational) from one's social network. Must know the types and the stress-buffering effect.
| Type | Description |
|---|---|
| Emotional | Empathy, love, trust, care |
| Instrumental | Tangible aid, goods, services |
| Informational | Advice, guidance, information |
| Appraisal | Feedback useful for self-evaluation |
Social support robustly improves health (lower cardiovascular disease, better immune function, lower mortality). Two competing models: the buffering hypothesis (support protects specifically by moderating stress's effects) vs. the direct effect hypothesis (support helps regardless of stress level). Notably, the perception of available support may matter more than support actually received.
Passage-levelSupport has a sociological dimension — networks, SES, and social capital determine who can access support, producing systematic health disparities.
Quick check: A cancer patient feels her symptoms are less overwhelming after joining a support group, even on days her objective health is unchanged. What form of support, and which hypothesis?
Answer: Emotional support is most prominent; reduced perceived burden despite unchanged stressors illustrates the buffering hypothesis — support is moderating the stressor's impact.
Biological Explanations of Social Behavior in Animals
Know the logicBehaviors that increase reproductive fitness are selected over generations. Apply natural-selection reasoning to social behaviors rather than just describing them.
Key mechanisms:
- Natural selection favors traits raising reproductive success; most social behaviors evolved because they ultimately raised fitness, even when costly on the surface.
- Sexual selection favors traits that raise mating success — by outcompeting rivals (intrasexual) or being preferred by the opposite sex (intersexual/mate-choice).
- Kin selection and reciprocal altruism explain apparently selfless behavior (below).
Hormones are the proximate mediators: oxytocin promotes bonding, trust, and affiliation; vasopressin supports pair bonding (prairie vole model); cortisol mediates stress moderated by social support.
Quick check: Male birds with more elaborate tail feathers father more offspring despite greater predation risk. What mechanism explains the tail's persistence?
Answer: Intersexual selection (mate-choice) — females prefer elaborate tails; the reproductive advantage outweighs the survival cost.
Foraging Behavior
Know the logicOptimal foraging theory (OFT): natural selection shapes foraging to maximize net energy gain per unit time; better foragers leave more energy for reproduction. Know the logic of two predictions:
- Prey choice: prefer prey maximizing net energy return — be selective when high-quality prey is abundant, broaden the diet when it's scarce.
- Patch use (Marginal Value Theorem): leave a patch when its local energy-gain rate falls to the average rate available in the habitat. Animals stay longer in good patches and leave sooner when travel between patches is quick.
Quick check: A forager picks berries; as easy berries deplete she must search harder. Per the Marginal Value Theorem, when should she move on?
Answer: When the current bush's caloric-gain rate falls to the average rate across all bushes in the area — leaving earlier wastes a productive patch, later wastes time in a depleted one.
Mating Behavior and Mate Choice
Parental Investment Theory
Must knowTrivers' parental investment theory — the sex that invests more in offspring is choosier; the sex that invests less competes more for mates. In most mammals, females invest more (gestation, lactation), so:
- Females are choosy (intersexual selection) — preferring good genes or resources.
- Males compete (intrasexual selection) — hence male-male aggression, dominance hierarchies, antlers.
Know the logic of female preferences: the good genes hypothesis (females prefer honest signals of genetic quality) and resource acquisition hypothesis (females prefer males with resources for offspring). Honest signals are costly and hard to fake (peacock's tail; Zahavi's handicap principle); symmetry is another cited signal.
Passage-levelsexual strategies theory (Buss & Schmitt) applies this to human short-term vs. long-term mating, predicting the sexes diverge most where investment differs. Mate choice copying = preferring a mate after seeing others choose them.
Mating Systems
Must know| System | Description | Typical condition |
|---|---|---|
| Monogamy | One male + one female | Both parents needed |
| Polygyny | One male + multiple females | Male can resource many females |
| Polyandry | One female + multiple males | Rare; low female investment relative to male |
| Promiscuity | Multiple partners, no bonds | Variable/seasonal |
Quick check: In a fish where males guard and fan the eggs while females leave after spawning, which sex is more selective, and why?
Answer: The males — they have higher parental investment (egg guarding), and per Trivers the higher-investing sex is choosier.
Applying Game Theory
Must knowGame theory analyzes strategic interactions where each actor's outcome depends on others' choices; in biology it explains the evolution of social strategies. (Out of scope: computing payoff matrices — reason about the games conceptually.)
The Prisoner's Dilemma
Must knowThe logic: two players each cooperate or defect. Mutual cooperation gives a moderate reward; mutual defection a small punishment; a lone defector gets the best payoff while the cooperator gets the worst. The "rational" move is always to defect, yet mutual defection beats neither — individually rational behavior yields collectively suboptimal outcomes. Models arms races, antibiotic resistance, overfishing, climate inaction.
The Hawk-Dove Game
Must knowHawk always escalates; Dove displays but retreats. A hawk invades a dove population easily; a dove invades a hawk population by avoiding injury. The result is a mixed evolutionarily stable strategy (ESS) — a stable ratio of hawk to dove behaviors that no pure strategy can invade.
An evolutionarily stable strategy (ESS) (Maynard Smith) is a strategy that, once common, cannot be invaded by a mutant alternative — it tells you what behavior selection favors.
Tit-for-Tat
Must knowIn iterated prisoner's dilemmas, cooperation can evolve. Axelrod's tournaments showed tit-for-tat (cooperate first, then copy the opponent's last move) succeeds: nice, retaliatory, forgiving, and clear. TFT underpins reciprocal altruism (next section).
Quick check: Two companies each decide whether to spend heavily on advertising (costly) or stay modest; if both advertise, share is unchanged but costs high; if neither does, costs low and share unchanged; if only one does, it gains share. Which model fits best?
Answer: The prisoner's dilemma — each has an incentive to defect (advertise) regardless of the other, yet mutual defection leaves both worse off than mutual cooperation.
Altruism and Inclusive Fitness
From a strict individual-selection view, true altruism — sacrificing your own fitness for another — should be eliminated. How can it persist?
Inclusive Fitness and Hamilton's Rule
Must knowHamilton's answer is inclusive fitness — selection favors maximizing copies of your genes passed on, including those passed via relatives who share them. This drives kin selection: a gene for altruism can spread if beneficiaries share that gene by descent. Hamilton's Rule (conceptual, not computed): altruism is favored when relatedness × benefit to the recipient exceeds the cost to the actor. You predict directionally — altruism is likelier toward close relatives than distant ones — never compute the inequality.
The coefficient of relatedness is the lever: roughly 0.5 for full siblings and parent-offspring, 0.25 for half-siblings, 0.125 for first cousins. (Haldane's quip — "I'd die for two brothers or eight cousins" — captures the same logic.)
Optionaleusocial insects (bees, ants) are the textbook case — via haplodiploidy, worker bees share ~0.75 relatedness with sisters (more than with their own daughters), so sterile worker behavior is favored.
Reciprocal Altruism
Must knowReciprocal altruism (Trivers) explains cooperation between unrelated individuals — help now, repaid later. It evolves when individuals interact repeatedly, can recognize each other (to exclude cheaters), and the recipient's benefit exceeds the donor's cost. Vampire bats sharing blood are the classic example. It is the biological analogue of tit-for-tat.
Optionalgroup selection (traits evolving for group benefit despite individual cost) is now considered weak relative to kin selection and reciprocal altruism, which are the accepted primary explanations.
Quick check: A ground squirrel gives an alarm call (exposing itself) more readily when close relatives are nearby than when only unrelated neighbors are. Which mechanism best explains this?
Answer: Kin selection — the call costs personal fitness but benefits relatives. Because relatedness is high for kin, Hamilton's rule is satisfied (relatedness-weighted benefit exceeds cost) only when relatives are present.
Common Confusions & Tricks
Proximity ≠ similarity. Proximity = physical closeness/repeated contact; similarity = shared attitudes/values. Both predict attraction, but proximity works even without similarity, and the mere exposure effect is the mechanism behind proximity — don't conflate the three.
Altruism (biological) ≠ altruism (psychological). Evolutionary altruism is defined purely by fitness cost/benefit (no motivation assumed); psychological altruism is defined by motivation. A behavior can be evolutionarily altruistic yet psychologically selfish, or vice versa — check which discipline the passage invokes.
Kin selection ≠ group selection. Kin selection acts on genes shared by relatives (inclusive fitness); group selection requires traits to spread for whole-group benefit regardless of relatedness — much weaker. If the answer involves relatedness, Hamilton's rule, or relatives, it's kin selection.
Diffusion of responsibility ≠ pluralistic ignorance. Diffusion = "someone else will do it" (reduced obligation). Pluralistic ignorance = "no one looks concerned, so it's probably not an emergency" (misreading the situation). Read for whether the character redefines the situation vs. passes off responsibility.
Ainsworth's avoidant child looks fine — but is insecurely attached. Little distress on separation and ignoring the caregiver on reunion can look like "independence"; it's insecure attachment — the child learned that expressing distress is futile.
Trivers' parental investment predicts selectivity, not passivity. The higher-investing sex is choosier, not weaker — female competition is real, just for access to high-quality mates rather than quantity.
Tit-for-tat is NOT forgiving on the first offense. TFT retaliates immediately (copies the last move); it is forgiving only in returning to cooperation once the opponent does. Not "turning the other cheek."
The Bobo doll experiment is Bandura, not Milgram or Zimbardo. Milgram = obedience (shocks). Zimbardo = Stanford Prison (roles/situational power). Bandura = children imitating aggression. Lock in the pairings.
Key Theories & Terms
| Term / Researcher | What It Means / Who They Are |
|---|---|
| Mere exposure effect | Repeated exposure increases positive evaluation, even unconsciously (Zajonc). |
| Halo effect | A positive impression in one domain (e.g., attractiveness) spreads to others (e.g., intelligence). |
| Matching hypothesis | Partners tend to be of roughly equivalent attractiveness. |
| Triangular theory of love | Sternberg: love = intimacy + passion + commitment. |
| Passionate vs. companionate love | Passionate = intense, arousal-driven (early); companionate = enduring affection/commitment. |
| Social exchange theory | Relationships judged by rewards minus costs, vs. comparison level and comparison level for alternatives. |
| Equity theory | Satisfaction depends on perceived fairness of each partner's benefit-to-contribution ratio. |
| Safe haven / secure base | Bowlby: attachment figure as comfort to return to / base to explore from. |
| Sexual strategies theory | Buss & Schmitt: sexes diverge in short- vs. long-term mating, following parental investment. |
| Frustration-aggression hypothesis | Dollard: frustration drives aggression; Berkowitz adds a required aggressive cue. |
| Weapons effect | A weapon (aggressive cue) increases aggressive responses. |
| Social learning theory / Bandura | Aggression learned via observation/modeling; shown by the Bobo doll experiment. |
| Excitation transfer theory | Zillmann: arousal from one source is misattributed to another, amplifying aggression. |
| Harlow's contact comfort | Rhesus monkeys preferred a cloth (comfort) over wire (food) surrogate. |
| Bowlby, John | Attachment as an evolved proximity-seeking system; internal working models. |
| Internal working model | Representations of self/others from early attachment that guide future relationships. |
| Ainsworth, Mary / Strange Situation | Standardized separations/reunions classifying secure, anxious/ambivalent, avoidant, disorganized attachment. |
| Bystander effect | Helping probability drops as bystanders increase (Latané & Darley). |
| Diffusion of responsibility | Each bystander feels less obligated because others are present. |
| Pluralistic ignorance | Ambiguous situation read as a non-emergency because others appear calm. |
| Empathy-altruism hypothesis | Batson: genuine empathy produces genuinely altruistic motivation. |
| Buffering hypothesis | Social support protects health mainly by moderating stress. |
| Optimal foraging theory (OFT) | Forage to maximize net energy gain per unit time; predicts diet breadth and patch-leaving. |
| Marginal Value Theorem | Leave a patch when local gain rate equals the habitat average. |
| Parental investment theory | Trivers: the higher-investing sex is choosier; the lower-investing sex competes. |
| Intersexual / intrasexual selection | Mate choice (usually female choosing) vs. within-sex competition (usually male-male). |
| Honest signal / handicap principle | Zahavi: reliable quality signals are costly and hard to fake (peacock tail). |
| Prisoner's dilemma | Individual rationality (defect) yields collectively suboptimal outcomes. |
| Hawk-dove game | Conflict over resources; predicts a mixed ESS of escalating vs. displaying. |
| Evolutionarily stable strategy (ESS) | Maynard Smith: once common, cannot be invaded by a mutant strategy. |
| Tit-for-tat (TFT) | Cooperate first, then copy the opponent's last move (Axelrod). |
| Inclusive fitness | Hamilton: personal reproduction plus relatives' reproduction, weighted by relatedness. |
| Hamilton's Rule | Altruism favored when relatedness × benefit > cost (conceptual, not computed). |
| Kin selection | Selection favoring altruism toward relatives because it raises inclusive fitness. |
| Coefficient of relatedness | ~0.5 full siblings/parent-offspring, 0.25 half-siblings, 0.125 first cousins. |
| Reciprocal altruism | Trivers: cooperation among non-relatives when interactions repeat and cheaters can be excluded. |
| Oxytocin | Neuropeptide promoting bonding, trust, and affiliation. |