Biological Bases of Behavior
The nervous system, neuron structure and the action potential, neurotransmitters, brain regions, and the genetic and evolutionary foundations of behavior.
AP Psychology begins with biology because all behavior — thought, emotion, memory, motivation — is ultimately a product of the nervous system. Understanding the neuron and how signals propagate is the foundation for everything else in the course.
The Nervous System
The nervous system is divided hierarchically:
- Central nervous system (CNS): brain and spinal cord — integration and command.
- Peripheral nervous system (PNS): all nerves outside the CNS.
- Somatic PNS: voluntary movement and sensory input.
- Autonomic PNS: involuntary organs (heart, digestion).
- Sympathetic: "fight or flight" — accelerates heart rate, dilates pupils, inhibits digestion.
- Parasympathetic: "rest and digest" — slows heart, constricts pupils, stimulates digestion.
Neurotransmitters — the Key Ones
Neurotransmitters are chemical messengers released from synaptic vesicles that cross the synapse and bind to receptor sites on the receiving neuron. Excess NT is reabsorbed by reuptake.
- Dopamine: reward, motivation, movement — deficit → Parkinson's; excess → schizophrenia.
- Serotonin: mood, sleep, appetite — low levels linked to depression.
- Norepinephrine: arousal, alertness — low levels linked to depression.
- Acetylcholine (ACh): muscle contraction, memory — deficit → Alzheimer's disease.
- GABA: main inhibitory NT — reduces neural firing; alcohol amplifies GABA.
- Glutamate: main excitatory NT — increases neural firing; involved in learning and memory.
- Endorphins: natural opioids — pain relief, euphoria (runner's high).
Brain Structures
The brain is organized from older evolutionary structures (brainstem) to newer (cortex). The limbic system handles emotion and memory.
- Brainstem (medulla, pons, reticular formation): automatic survival functions — heartbeat, breathing, arousal.
- Cerebellum: coordination, balance, procedural memory.
- Hypothalamus: hunger, thirst, body temperature, sex drive — the "4 F's" (feeding, fighting, fleeing, mating).
- Amygdala: emotion processing — especially fear and aggression; threat detection.
- Hippocampus: explicit memory formation — damage causes anterograde amnesia (cannot form new long-term memories).
- Thalamus: sensory relay station — routes sensory information to appropriate cortex areas.
- Prefrontal cortex: executive function, planning, impulse control, judgment.
- Left hemisphere: language, logic, sequential processing (Broca's area: speech production; Wernicke's area: language comprehension).
- Right hemisphere: spatial reasoning, facial recognition, holistic processing.
Click through each phase of the action potential to see how Na⁺ and K⁺ channels open and close, driving the membrane voltage from −70 mV to +40 mV and back. The chart shows the classic action potential curve with the threshold, peak, repolarization, and refractory period labeled.
The neuron is at rest. Na⁺/K⁺-ATPase pumps maintain a −70 mV resting potential. Na⁺ channels are closed; K⁺ channels are closed.
Resting: high K⁺ inside, high Na⁺ outside — maintained by the sodium-potassium pump.
Key Concepts
Exam tip: The FRQ will often give you a brain injury scenario and ask which structures were damaged based on the behavioral symptoms. Remember: hippocampus = memory formation (can't make new LTM), amygdala = fear/emotion, prefrontal cortex = planning/impulse control, Broca's area (left frontal) = speech production, Wernicke's area (left temporal) = language comprehension, cerebellum = coordination. Phineas Gage (prefrontal damage → personality change) is a classic FRQ setup.
Common mistake: Do not confuse the amygdala (emotion — especially fear) with the hippocampus (memory formation). They are adjacent in the limbic system but have distinct functions. A person with hippocampal damage remembers the past but cannot form new memories. A person with amygdala damage has blunted emotional responses and impaired fear conditioning — but can form new declarative memories.
Cognition
Sensation, perception, memory systems, encoding and retrieval, forgetting, language, thinking, heuristics and biases, intelligence, and consciousness.
Cognition is the largest unit — it covers how we take in information (sensation and perception), store and retrieve it (memory), think with it (reasoning and problem-solving), and express it (language). This unit accounts for over a quarter of the exam.
Sensation vs. Perception
- Sensation: detection of physical energy by sense organs — the raw data.
- Perception: interpretation of sensory data — organized, meaningful experience.
- Absolute threshold: minimum stimulus detected 50% of the time.
- Difference threshold (JND): smallest detectable change — governed by Weber's Law: ΔI/I = k (constant ratio).
- Signal detection theory: detecting a stimulus depends on both its intensity and the observer's response bias (d-prime and criteria).
Gestalt Principles of Perception
The brain organizes sensory input into meaningful wholes (gestalts): figure-ground (distinguish object from background), proximity (nearby elements grouped), similarity (similar elements grouped), continuity (prefer smooth, continuous patterns), closure (complete incomplete shapes).
Sleep and Consciousness
Sleep has distinct stages detectable by EEG. NREM stages 1–3 progress from light to deep sleep; REM sleep features rapid eye movements, vivid dreaming, and paradoxical activation of the brain with muscle paralysis. A full sleep cycle is ~90 minutes. Sleep deprivation impairs memory consolidation — the hippocampus replays experiences during sleep to transfer memories to cortex.
Key Concepts
Select an encoding strategy, then click 'Start Encoding' to watch words flow through the sensory store → short-term memory → long-term memory. With no rehearsal, most words are lost before reaching LTM. Elaborative encoding dramatically increases retention. The forgetting curve below shows Ebbinghaus's finding that without review, memories decay exponentially — and how deeper encoding slows that decay.
Items enter sensory store briefly, a few reach STM, but without rehearsal they fade after ~20 seconds. Little reaches LTM.
Sensory Store
~0.5–3 sec · unlimited
Short-Term Memory
~20 sec · 4 chunks
Long-Term Memory
Unlimited · days-lifetime
Ebbinghaus Forgetting Curve — by encoding strategy
The forgetting curve (Ebbinghaus, 1885) shows exponential decay without rehearsal. Elaborative encoding and spaced repetition dramatically slow forgetting. STM capacity ≈ 7±2 chunks (Miller, 1956).
Memory Systems
The Atkinson–Shiffrin multi-store model describes three memory stores:
- Sensory memory: very brief (0.5–3 sec), unlimited capacity — iconic (visual) and echoic (auditory).
- Short-term memory (STM): ~20 seconds, 7±2 chunks (Miller's Law). Maintained by rehearsal.
- Long-term memory (LTM): potentially unlimited capacity and duration.
Craik and Lockhart's levels of processing theory argues that what matters is the depth of encoding, not which store information enters. Shallow encoding (visual/acoustic) produces weaker memories; deep semantic encoding (meaning, elaboration) produces stronger, more durable memories.
Types of Long-Term Memory
- Explicit (declarative): conscious recollection — episodic (personal events) and semantic (facts, concepts). Requires hippocampus.
- Implicit (nondeclarative): unconscious — procedural skills (riding a bike), priming, conditioned responses. Does NOT require hippocampus.
Forgetting
- Decay theory: memories fade with time if not rehearsed.
- Interference: proactive (old learning interferes with new) and retroactive (new learning interferes with old).
- Retrieval failure: information is stored but retrieval cues are absent (tip-of-the-tongue state).
Exam tip: The FRQ will describe a person failing to remember something and ask you to explain why using memory concepts. Have four explanations ready: (1) encoding failure — never reached LTM (no deep processing); (2) storage decay — faded over time; (3) proactive interference — older memories block recall; (4) retroactive interference — newer learning blocks recall. Name the specific concept, don't just describe it.
Common mistake: Do not confuse proactive and retroactive interference. Proactive = old interferes with new (you learned Spanish first, now it keeps coming up when you try to learn French). Retroactive = new interferes with old (learning French makes it harder to recall your Spanish). 'Pro' = forward; 'retro' = backward.
Development and Learning
Physical and cognitive development across the lifespan, Piaget's stages, attachment theory, classical conditioning, operant conditioning, and observational learning.
This unit covers how we change across the lifespan (development) and how experience modifies behavior (learning). These are among the most heavily tested topics on both the multiple-choice and FRQ sections.
Piaget's Stages of Cognitive Development
- Sensorimotor (0–2 years): learning through senses and motor actions; key achievement: object permanence (~8 months) — objects exist when out of sight.
- Preoperational (2–7 years): language emerges; symbolic thinking; lacks conservation (same amount looks different in different containers); egocentrism (can't take another's perspective).
- Concrete Operational (7–12 years): logical thinking about concrete objects; masters conservation; understands reversibility; can categorize.
- Formal Operational (12+): abstract and hypothetical reasoning; systematic problem-solving; moral reasoning develops.
Vygotsky vs. Piaget
Piaget emphasized independent discovery; Vygotsky emphasized the role of social interaction and language in cognitive development. Vygotsky's zone of proximal development (ZPD): the range between what a child can do independently and what they can do with guidance — this is the optimal teaching zone.
Attachment — Ainsworth's Strange Situation
Mary Ainsworth identified three (later four) attachment styles: secure (distressed when parent leaves, comforted when returns — 65% of children), anxious/ambivalent (highly distressed, not calmed by return), avoidant (no distress; ignores parent). Secure attachment predicts better social outcomes. Harlow's monkey studies showed that contact comfort, not feeding, drives attachment.
Erikson's Psychosocial Stages
Erikson proposed 8 stages across the lifespan, each with a central conflict. Key stages: Trust vs. Mistrust (infancy), Initiative vs. Guilt (preschool), Industry vs. Inferiority (school age), Identity vs. Role Confusion (adolescence — the most-tested), Intimacy vs. Isolation (young adulthood), Integrity vs. Despair (late adulthood).
Switch between tabs to explore both major learning theories. Classical conditioning: step through acquisition, extinction, and spontaneous recovery to see how a conditioned response (CR) forms and fades. Operant conditioning: compare the four reinforcement schedules (FR, VR, FI, VI) side by side — variable ratio produces the highest response rates and greatest resistance to extinction.
CS
Bell
present
UCS
Food
absent
Response
Salivation
5%
CS alone (e.g. bell) produces no meaningful response. UCS (food) produces UCR (salivation) naturally.
Classical Conditioning — Pavlov
Ivan Pavlov discovered that a neutral stimulus (bell = CS) paired with an unconditioned stimulus (food = UCS) eventually elicits a conditioned response (salivation = CR). Key phenomena:
- Acquisition: CS and UCS are repeatedly paired; CR strengthens.
- Extinction: CS presented without UCS; CR weakens. Not erased — only suppressed.
- Spontaneous recovery: after rest, extinguished CR reappears at reduced strength.
- Generalization: similar stimuli elicit CR (Watson's Little Albert — fear of white rat generalized to white rabbit, fur coat).
- Discrimination: only the original CS elicits CR; similar stimuli do not.
Operant Conditioning — Skinner
B.F. Skinner showed that behavior is shaped by its consequences. A reinforcer increases the frequency of behavior; a punisher decreases it.
- Positive reinforcement: add a pleasant stimulus (give candy for good behavior).
- Negative reinforcement: remove an aversive stimulus (take painkillers removes headache → reinforces pill-taking).
- Positive punishment: add an aversive stimulus (speeding ticket).
- Negative punishment: remove a pleasant stimulus (take away phone).
Observational Learning — Bandura
Albert Bandura's Bobo doll experiment showed that children imitate aggressive behavior observed in adults — without direct reinforcement. Key processes: attention, retention, reproduction, motivation. Bandura also introduced self-efficacy: belief in one's ability to perform a behavior — a key predictor of actual performance.
Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might select.
— John B. Watson, Behaviorism, 1924
Why it matters: Watson's radical behaviorist claim — that environment (not biology) shapes all behavior — launched the conditioning era. His Little Albert experiment demonstrated fear conditioning in humans. AP FRQs often ask you to evaluate claims like this using both classical conditioning evidence and nature-nurture counterarguments.
Key Concepts
Exam tip: The FRQ most commonly asks you to apply conditioning concepts to a new scenario. For operant conditioning, always identify: (1) the behavior being targeted, (2) whether you are using reinforcement (+/-) or punishment (+/-), and (3) what specific stimulus is added or removed. The words 'positive' and 'negative' mean add/remove — not good/bad. Negative reinforcement is always reinforcement (behavior increases).
Common mistake: Students consistently confuse negative reinforcement with punishment. Negative reinforcement INCREASES behavior by removing something unpleasant. Punishment DECREASES behavior. If a teenager cleans their room to stop their parent nagging — that is negative reinforcement (cleaning increased, nag removed). If the parent takes away their phone for failing a test — that is negative punishment (studying increased, phone removed).
Social Psychology & Personality
Attitudes, conformity, obedience, group behavior, attribution, prejudice, personality theories, motivation, and emotion.
Social psychology examines how people's thoughts, feelings, and behaviors are influenced by others — both real and imagined presence. Some of the most famous experiments in all of psychology come from this unit.
The Power of the Situation — Classic Experiments
- Asch conformity study: ~75% of participants gave at least one wrong answer on a line-judging task to conform to group consensus — even with no reward and no threat. Shows normative social influence (desire to fit in).
- Milgram obedience study: 65% of participants administered what they believed were 450-volt shocks to strangers when instructed by an authority figure. Shows the power of legitimate authority and situational pressure.
- Zimbardo's Stanford Prison Experiment: Normal college students assigned to guard/prisoner roles began to enact those roles — showing how situational roles override individual character.
- Bystander effect (Latané & Darley): the more people present in an emergency, the less likely any individual helps — due to diffusion of responsibility.
Attribution Theory
Attribution is how we explain others' (and our own) behavior:
- Fundamental attribution error (FAE): overattributing others' behavior to disposition (character) and underweighting the situation. "He's a rude person" rather than "he's having a rough day."
- Actor-observer bias: we explain our own behavior situationally ("I failed because the test was hard") but others' behavior dispositionally ("they failed because they're not smart").
- Self-serving bias: attribute successes to internal factors (ability, effort) and failures to external factors (bad luck, unfair test).
Personality Theories
AP Psychology covers four major personality frameworks:
- Psychoanalytic (Freud): behavior driven by unconscious conflicts; id/ego/superego; defense mechanisms (repression, projection, rationalization, displacement).
- Humanistic (Rogers, Maslow): people are inherently good and motivated toward growth. Rogers: unconditional positive regard promotes healthy self-concept. Maslow: hierarchy of needs (physiological → safety → love → esteem → self-actualization).
- Trait theories (Big Five — OCEAN): Openness, Conscientiousness, Extraversion, Agreeableness, Neuroticism — relatively stable across the lifespan.
- Social-cognitive (Bandura): reciprocal determinism — behavior, personal factors, and environment all mutually influence each other.
The most important thing I have learned from many years of research is that the most significant factor in human behavior is the power of the situation.
— Philip Zimbardo, The Lucifer Effect, 2007
Why it matters: Zimbardo's situationist conclusion — supported by Milgram, Asch, and the Stanford Prison Experiment — is a recurring AP exam theme. FRQs often ask you to distinguish dispositional (trait) from situational explanations for behavior, and to identify the fundamental attribution error.
Key Concepts
Exam tip: The FRQ will often describe a social situation and ask you to identify the psychological concept at work. Master these pairings: large group + emergency + no one helps = bystander effect + diffusion of responsibility; authority figure + harmful orders followed = Milgram/obedience; group gives wrong consensus → individual agrees = Asch/conformity; explaining someone's rude behavior as their character = fundamental attribution error.
Common mistake: Do not confuse conformity, compliance, and obedience — they represent a spectrum of social influence. Conformity: going along with group norms without being directly asked (Asch). Compliance: changing behavior in response to a direct request from a peer (e.g., foot-in-the-door technique). Obedience: changing behavior in response to commands from an authority figure (Milgram). Always specify which one the scenario demonstrates.
Mental and Physical Health
Stress and coping, psychological disorders (DSM-5), major treatment approaches — psychodynamic, behavioral, cognitive-behavioral, humanistic, and biological.
This unit covers how psychology intersects with health — from how stress affects the body to how we define, diagnose, and treat psychological disorders. The biopsychosocial model frames all disorders as arising from the interaction of biological, psychological, and social factors.
Stress — Selye's General Adaptation Syndrome
Hans Selye identified a three-stage physiological response to prolonged stress:
- Alarm stage: initial "fight-or-flight" — cortisol and adrenaline release, heart rate increases, immune function temporarily suppressed.
- Resistance stage: body adapts; hormonal levels stabilize at elevated state; maintains heightened readiness.
- Exhaustion stage: prolonged stress depletes resources — increased vulnerability to illness, burnout, and physical breakdown.
Lazarus added a cognitive component: how you appraise a situation determines whether it becomes a stressor. Primary appraisal (is this threatening?) and secondary appraisal (can I cope?) shape the stress response.
Psychological Disorders — Key Categories
- Anxiety disorders: persistent, excessive fear — generalized anxiety disorder (GAD), panic disorder, specific phobias, social anxiety, OCD, PTSD.
- Depressive disorders: major depressive disorder (MDD) — depressed mood, loss of interest, at least 2 weeks.
- Bipolar disorders: episodes of mania (elevated mood, decreased need for sleep, impulsivity) alternating with depression.
- Schizophrenia spectrum: positive symptoms (hallucinations, delusions, disorganized speech) and negative symptoms (flat affect, avolition, alogia); excess dopamine hypothesis.
- Dissociative disorders: disruptions in consciousness, memory, identity — dissociative identity disorder (formerly multiple personality disorder).
- Personality disorders: inflexible, maladaptive traits across contexts — antisocial PD (disregard for others), borderline PD (emotional instability), narcissistic PD.
Treatment Approaches
- Psychodynamic therapy (Freud): bring unconscious conflicts into awareness; free association, dream analysis, transference.
- Humanistic therapy (Rogers): person-centered therapy; unconditional positive regard; active listening; builds self-esteem.
- Behavioral therapy: applies learning principles — systematic desensitization (exposure + relaxation for phobias), token economies (operant conditioning).
- Cognitive-behavioral therapy (CBT): identifies and challenges distorted cognitions (Beck's cognitive triad: self, world, future). Most evidence-supported treatment for depression and anxiety.
- Biomedical: antidepressants (SSRIs — serotonin reuptake inhibition), antipsychotics (block dopamine receptors), mood stabilizers (lithium for bipolar), anxiolytics (benzodiazepines — amplify GABA).
The greatest discovery of any generation is that a human can alter his life by altering his attitude.
— William James, Principles of Psychology, 1890
Why it matters: James's insight anticipates cognitive-behavioral therapy — that how we think shapes how we feel and behave. CBT explicitly operationalizes this: change maladaptive cognitions → change emotions → change behavior. AP FRQs on treatment often require you to explain why CBT is effective using this cognitive model.
Key Concepts
Exam tip: FRQs on treatment often ask you to select and justify a treatment for a specific disorder. CBT is the safe all-purpose answer for depression and anxiety — but be ready to explain: (1) what cognitive distortions it targets, (2) what behavioral component it includes. For phobias specifically, systematic desensitization and exposure therapy are the most-cited behavioral techniques. For schizophrenia, biomedical (antipsychotics) is primary.
Common mistake: Do not confuse the symptoms of MDD with bipolar disorder. Both involve depressive episodes — but bipolar disorder also requires at least one manic episode (elevated mood, grandiosity, decreased need for sleep, reckless behavior). Treating bipolar with only antidepressants (without mood stabilizers) can trigger a manic episode. This distinction is a common FRQ trap.
Exam prediction: This topic frequently appears on the AP Psychology exam. See our full AP Psychology predictions →
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