Interaction of Heredity and Environment
Explains how nature and nurture interact using twin studies, adoption studies, and the evolutionary perspective to understand behavior and mental processes.
AP Psychology Unit 1 Review: Biological Bases of Behavior
- 15–25% of AP multiple choice
- 6 topics
- AP exam review verified for 2027
Unit 1 covers the biological systems that underlie all behavior: heredity-environment interaction, nervous system organization, neuron structure and neural firing, brain anatomy and function, sleep and consciousness, and sensation across all sensory systems.
Fiveable's AP Psychology Unit 1 review covers 6 topics, with guided practice questions for the unit.
AP Psychology unit 1 topics
Overview of the Nervous System
Covers the CNS and PNS, including the somatic, autonomic, sympathetic, and parasympathetic divisions and their voluntary and involuntary functions.
The Neuron and Neural Firing
Explains neuron structure, the action potential, the all-or-none principle, neurotransmitter function, and how psychoactive drugs alter neural communication.
The Brain
Maps brain structures to their functions, from the brain stem to the cortical lobes, and covers split-brain research, language areas, and neuroplasticity.
Sleep
Covers circadian rhythms, NREM and REM sleep stages identified by EEG, theories of dreaming, REM rebound, and common sleep disorders.
Sensation
Explains transduction, absolute threshold, Weber's law, and the structures and functions of all major sensory systems including vision, hearing, taste, smell, touch, pain, vestibular, and kinesthesis.
Practice AP Psychology unit 1 questions
Try AP-style questions on unit 1, then use the review notes below for anything you miss.
AP Psychology unit 1 practice test
A timed, AP-style test built only from unit 1 questions — multiple choice plus free response, scored with AI feedback. Use the standard format or build your own.
Unit 1 review notes
1.1
Heredity and Environment
Behavior and mental processes result from the interaction of genetic predispositions (nature) and environmental experiences (nurture). Neither factor acts alone. Researchers use twin studies, adoption studies, and family studies to estimate how much each contributes to a given trait. The evolutionary perspective argues that natural selection has shaped behaviors that increase survival and reproductive success, though some historical applications of this idea, such as eugenics, have been used to discriminate.
- Nature (heredity): Genetic or predisposed characteristics that influence physical, behavioral, and mental traits.
- Nurture (environment): External factors such as family interactions, education, and culture that shape development.
- Twin studies: Compare monozygotic (identical) and dizygotic (fraternal) twins to estimate heritability of traits.
- Adoption studies: Compare adopted children with biological and adoptive parents to separate genetic from environmental influences.
- Evolutionary perspective: Explains behavior as shaped by natural selection to increase survival and reproductive success.
Can you explain why twin and adoption studies are useful for separating genetic from environmental contributions to a trait?
| Method | What it compares | What it reveals |
|---|---|---|
| Twin studies | Identical vs. fraternal twins | Heritability of traits |
| Adoption studies | Adopted child vs. biological and adoptive parents | Genetic vs. environmental influence |
| Family studies | Relatives of varying genetic closeness | How trait similarity tracks with genetic relatedness |
1.2
Organization of the Nervous System
The nervous system divides into the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which connects the CNS to the rest of the body. The PNS splits into the somatic nervous system, controlling voluntary skeletal movement, and the autonomic nervous system, controlling involuntary processes. The autonomic system further divides into the sympathetic branch, which activates the body for stress responses, and the parasympathetic branch, which returns the body to a resting state.
- Central nervous system (CNS): Brain and spinal cord; the command center for all behavior and mental processes.
- Somatic nervous system: Controls voluntary skeletal muscle movement.
- Autonomic nervous system: Controls involuntary processes such as heart rate and digestion.
- Sympathetic nervous system: Activates the fight-or-flight response during stress.
- Parasympathetic nervous system: Restores the body to a calm, resting state after arousal.
Trace the path from the CNS outward: what does each branch of the peripheral nervous system control?
| Branch | Voluntary or involuntary | Example function |
|---|---|---|
| Somatic | Voluntary | Moving your hand |
| Autonomic: Sympathetic | Involuntary | Increased heart rate during stress |
| Autonomic: Parasympathetic | Involuntary | Slowed heart rate at rest |
1.3
Neurons, Neural Firing, and Psychoactive Drugs
Neurons are the basic signaling units of the nervous system. They receive input through dendrites, process it in the cell body, and send signals down the axon to the terminal buttons. Glial cells support neurons by providing structure, insulation via myelin, and waste transport. When a neuron reaches threshold, it fires an action potential following the all-or-none principle. The signal crosses the synapse chemically via neurotransmitters, which can be excitatory or inhibitory. Psychoactive drugs alter this process: agonists mimic or enhance neurotransmitter effects, antagonists block them, and reuptake inhibitors prevent neurotransmitters from being reabsorbed.
- All-or-none principle: A neuron fires completely at full strength or not at all once threshold is reached.
- Reflex arc: A spinal pathway using sensory neurons, interneurons, and motor neurons to produce rapid automatic responses.
- Neurotransmitter: Chemical messenger released into the synapse; examples include dopamine, serotonin, GABA, and glutamate.
- Agonist: A drug or substance that enhances or mimics a neurotransmitter's effect, increasing neural firing.
- Reuptake inhibitor: Blocks reabsorption of a neurotransmitter back into the sending neuron, prolonging its effect in the synapse.
What is the difference between an agonist and an antagonist, and how does each affect neural firing?
| Drug type | Effect on neural firing | Example |
|---|---|---|
| Stimulant | Increases neural activity | Cocaine, caffeine |
| Depressant | Decreases neural activity | Alcohol |
| Hallucinogen | Distorts perception and cognition | Marijuana |
| Opioid | Reduces pain; mimics endorphins | Heroin |
| Reuptake inhibitor | Prolongs neurotransmitter action | SSRIs (serotonin) |
1.4
Brain Structures and Functions
Different brain regions handle different functions, and damage to a region predicts specific behavioral or cognitive changes. The brain stem and medulla regulate basic survival functions. The cerebellum coordinates movement and balance. The limbic system structures, including the hippocampus, amygdala, thalamus, and hypothalamus, handle memory, emotion, sensory relay, and homeostasis. The cerebral cortex is divided into four lobes, each with distinct functions. The corpus callosum connects the two hemispheres, and split-brain research reveals that the hemispheres have specialized roles. Neuroplasticity means the brain can reorganize in response to experience or injury.
- Brain stem / medulla: Controls basic life functions: breathing, heart rate, and blood pressure.
- Hippocampus: Critical for forming new long-term memories; part of the limbic system.
- Amygdala: Processes emotion, especially fear and threat detection.
- Split-brain research: Studies of patients with severed corpus callosum showing that left and right hemispheres have specialized functions.
- Aphasia: Language impairment from brain damage; Broca's area damage disrupts speech production, Wernicke's area damage disrupts comprehension.
If a patient has damage to the temporal lobe, what specific functions would be most affected?
| Brain region | Primary function |
|---|---|
| Medulla / brain stem | Breathing, heart rate |
| Cerebellum | Motor coordination, balance, procedural learning |
| Hippocampus | Memory formation |
| Occipital lobe | Visual processing |
| Frontal lobe | Decision-making, motor control, personality |
1.5
Sleep and Consciousness
Sleep is a form of consciousness that follows a circadian rhythm of roughly 24 hours, regulated by the suprachiasmatic nucleus and melatonin. Sleep stages are identified by EEG patterns. NREM Stages 1 through 3 progress from light sleep to slow-wave deep sleep. REM sleep is paradoxical because brain activity resembles wakefulness while the body is most relaxed; dreaming typically occurs here and increases in frequency across the night. When REM sleep is lost, REM rebound occurs. Theories of sleep function include activation-synthesis (dreams as the brain interpreting random signals), consolidation (sleep strengthens memory), and restoration (sleep repairs the body). Sleep disorders such as insomnia, narcolepsy, sleep apnea, and somnambulism each disrupt specific stages or functions.
- Circadian rhythm: The roughly 24-hour biological cycle that regulates sleep and wakefulness; disrupted by jet lag and shift work.
- REM sleep: Paradoxical sleep stage with wake-like EEG activity, muscle atonia, and frequent dreaming; increases across the night.
- REM rebound: Increased REM sleep following REM deprivation, suggesting REM serves a necessary function.
- Somnambulism: Sleepwalking; occurs during NREM slow-wave sleep, not REM.
- Activation-synthesis theory: Proposes that dreams result from the brain trying to make sense of random neural activity during REM sleep.
Why is REM sleep called paradoxical, and what happens to REM when a person is deprived of it?
| Stage | EEG pattern | Key feature |
|---|---|---|
| NREM Stage 1 | Theta waves | Hypnagogic sensations, light sleep |
| NREM Stage 2 | Sleep spindles, K-complexes | Deeper sleep, harder to wake |
| NREM Stage 3 | Delta waves | Slow-wave deep sleep, restoration |
| REM | Beta-like (wake-similar) | Dreaming, muscle atonia, memory consolidation |
1.6
Sensation Across Sensory Systems
Sensation is the detection of environmental stimuli above a threshold and their conversion into neural signals through transduction. The absolute threshold is the minimum stimulus intensity detected 50% of the time. The just-noticeable difference (JND) and Weber's law describe how much change is needed to notice a difference. Sensory adaptation occurs when prolonged exposure reduces sensitivity. Each sensory system, including vision, hearing, taste, smell, touch, pain, vestibular, and kinesthetic, has specialized structures that transduce specific energy types. Key examples include rods and cones in the retina, hair cells in the cochlea, and semicircular canals for balance. Smell is the only sense not first processed by the thalamus.
- Transduction: The conversion of physical stimuli (light, sound, pressure) into neural signals the brain can process.
- Absolute threshold: The minimum stimulus intensity that can be detected at least 50% of the time.
- Weber's law: The JND is a constant proportion of the original stimulus; larger stimuli require bigger changes to notice a difference.
- Place theory: Different pitches activate different locations along the basilar membrane of the cochlea.
- Kinesthesis: The sense of body movement and position that allows coordinated movement without visual monitoring.
How do trichromatic theory and opponent-process theory together explain color vision, including afterimages?
| Sensory system | Transduction structure | Notable feature |
|---|---|---|
| Vision | Rods and cones in the retina | Trichromatic and opponent-process theories explain color |
| Hearing | Hair cells on basilar membrane | Place, frequency, and volley theories explain pitch |
| Smell (olfaction) | Olfactory receptors in nose | Only sense bypassing the thalamus first |
| Taste (gustation) | Taste buds on tongue | Six basic tastes; supertasters have more receptors |
| Vestibular | Semicircular canals in inner ear | Detects rotational movement and balance |
Key terms
| Term | Definition |
|---|---|
| all-or-none principle | A neuron fires at full strength once threshold is reached, or it does not fire at all; the size of the action potential does not vary with stimulus intensity. |
| reflex arc | A spinal pathway in which sensory neurons, interneurons, and motor neurons work together to produce rapid automatic responses without brain involvement. |
| Interneurons | Neurons located entirely within the CNS that connect sensory and motor neurons, as in the spinal reflex arc. |
| Serotonin | A neurotransmitter that regulates mood, appetite, sleep, and memory; low levels are associated with depression. |
| Stimulants | Psychoactive drugs such as caffeine and cocaine that increase neural activity, producing heightened alertness and energy. |
| brain stem | The lower brain region including the medulla that controls basic survival functions such as breathing and heart rate. |
| Aphasia | A language impairment caused by brain damage; Broca's area damage disrupts speech production and Wernicke's area damage disrupts language comprehension. |
| split brain research | Studies of patients with a severed corpus callosum showing that the left hemisphere specializes in language and the right in spatial processing. |
| somnambulism | Sleepwalking; a sleep disorder involving complex motor behavior during NREM slow-wave sleep, not during REM. |
| Transduction | The conversion of physical stimulus energy (light, sound, pressure) into neural signals that the brain can interpret. |
| Synesthesia | A condition in which stimulation of one sensory pathway automatically triggers an experience in another, such as seeing colors when hearing music. |
| Place Theory | The theory that pitch perception depends on which specific location along the basilar membrane is most activated by a sound wave. |
| adoption studies | A research method comparing adopted children with biological and adoptive parents to separate genetic from environmental contributions to a trait. |
| semicircular canals | Fluid-filled inner ear structures that detect rotational head movements and serve as the primary receptors for the vestibular sense. |
| kinesthesis | The sense of body movement and position that allows coordinated movement without visually monitoring each body part. |
How to review unit 1
Topics students miss most
Miss rates come from Fiveable multiple-choice practice.
Common unit 1 mistakes
Treating nature and nurture as opposites
Students often write that a trait is caused by genes OR environment. The correct position is that heredity and environment always interact; twin and adoption studies estimate relative contributions, not exclusive causes.
Confusing sympathetic and parasympathetic effects
Remember that sympathetic activates (fight-or-flight: increased heart rate, dilated pupils) and parasympathetic restores (rest-and-digest: slowed heart rate, digestion resumes). Mixing these up on scenario questions costs points.
Misapplying the all-or-none principle
The all-or-none principle means a neuron fires at full strength or not at all, not that the nervous system responds in an all-or-nothing way. Intensity is coded by the rate and number of neurons firing, not by the size of individual action potentials.
Placing dreaming in NREM sleep
Dreaming typically occurs during REM sleep. Somnambulism (sleepwalking) occurs during NREM Stage 3. Students frequently swap these when answering sleep disorder questions.
Confusing absolute threshold with JND
Absolute threshold is the minimum intensity to detect a stimulus at all (50% of the time). The just-noticeable difference is the minimum change in an already-present stimulus that can be detected. These are separate concepts with separate definitions.
How this unit shows up on the AP exam
Applying biological concepts to behavior scenarios
AP Psychology questions frequently present a scenario (a patient with a brain injury, a person under stress, someone taking a drug) and ask you to identify which structure, neurotransmitter, or nervous system branch explains the described behavior. Practice moving from symptom to biological cause and from biological cause to predicted behavior.
Comparing research methods for heredity
Questions may ask you to evaluate what twin studies versus adoption studies can and cannot tell us about heritability. Be ready to explain what each method controls for and what conclusions are and are not supported by the data.
Explaining sensation processes with threshold concepts
Sensation questions often require you to apply absolute threshold, JND, or Weber's law to a specific example, or to explain how transduction works in a named sensory system. FRQ responses on sensation typically require you to define a concept and then connect it to a real-world behavior or perceptual outcome.
More ways to review
Topic study guides
Open the individual guides for Unit 1 when you want a closer review of one topic.
Practice questions
Use AP-style practice after you review the notes so you can check what you understand.
FRQ practice
Practice free-response reasoning and compare your answer with scoring guidance.
Cram archive videos
Watch past review streams filtered to Unit 1 when you want a video walkthrough.
Official unit cheatsheet
Open the Fiveable one-page unit review, then explore visual cheatsheets for a quick refresher.
Score calculator
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Unit 1 printables
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get printablesFrequently Asked Questions
What's on the AP Psych Unit 1 progress check (MCQ and FRQ)?
The AP Psych Unit 1 progress check in AP Classroom includes both MCQ and FRQ sections drawn from all 6 unit topics: Interaction of Heredity and Environment, Overview of the Nervous System, The Neuron and Neural Firing, The Brain, Sleep, and Sensation. MCQs test concept recognition across these topics, while the FRQ asks you to apply and connect them, often centering on how the brain or neural firing explains a behavior. For matched practice questions that mirror the progress check format, visit AP Psych Unit 1.
How do I practice AP Psych Unit 1 FRQs?
AP Psych Unit 1 FRQs most often ask you to explain how the brain, neuron and neural firing, or the interaction of heredity and environment connects to a real-world behavior or scenario. The question type is typically a concept application prompt where you define a term and then use it to explain something specific. To practice, write out short responses to scenario-based prompts for each of the 6 topics, then check that every sentence directly answers the prompt without drifting. You can find Unit 1 FRQ practice at AP Psych Unit 1.
Where can I find AP Psych Unit 1 practice questions?
The best place to find AP Psych Unit 1 practice questions, including multiple-choice and practice test sets, is AP Psych Unit 1. That page has MCQs and FRQs covering all 6 topics: the brain, neuron and neural firing, sleep, sensation, the nervous system, and heredity and environment. Running through unit-specific MCQ sets is the fastest way to spot which topics need more review before a full practice test.
How should I study AP Psych Unit 1?
Start AP Psych Unit 1 by building a clear mental map of how the brain and nervous system work before moving to the finer details. Here's a practical order that works well: 1. Interaction of Heredity and Environment first, since it frames why biology matters for behavior at all.
2. Overview of the Nervous System and The Neuron and Neural Firing together, so you understand how signals travel before you study where they go.
3. The Brain next, mapping major structures to their functions using labeled diagrams.
4. Sleep and Sensation last, applying what you know about neural firing to explain how these processes work. For each topic, write a one-sentence plain-English explanation, then test yourself with MCQs at AP Psych Unit 1. Since this unit is 15-25% of the exam, returning to it periodically throughout the year pays off.