Interaction of Heredity and Environment
The age-old question: Nature vs. Nurture. Modern psychology recognizes that behavior emerges from the interaction of both genetic predispositions and environmental influences.
Nature: Genetic inheritance—the biological factors passed from parents to offspring through DNA.
Nurture: Environmental influences—experiences, culture, parenting, education, and social interactions that shape who we become.
"Genes load the gun, environment pulls the trigger."
🧬 Twin Studies
Compare identical (monozygotic) vs. fraternal (dizygotic) twins to estimate heritability of traits.
👨👩👧 Adoption Studies
Compare adopted children to biological vs. adoptive parents to separate genetic from environmental effects.
- Know the difference: Genotype (genetic makeup) vs. Phenotype (observable characteristics).
- Heritability: The proportion of variation in a trait attributable to genetics (applies to populations, NOT individuals).
- Common FRQ topic: Explaining how BOTH nature AND nurture contribute to a specific behavior.
Research suggests intelligence is about 50-80% heritable. However, environmental factors like nutrition, education, and enrichment significantly impact IQ scores.
The Flynn Effect—rising IQ scores over generations—demonstrates environmental influence since genes haven't changed that quickly.
1) Define heritability and explain why a trait being "highly heritable" doesn't mean it's unchangeable.
2) How would a researcher use twin studies to investigate the genetic basis of depression?
Show Answers
1) Heritability is the proportion of variation in a trait within a population attributable to genetic differences. Even highly heritable traits can be modified by environment (e.g., height is highly heritable but influenced by nutrition).
2) Compare concordance rates (both twins having depression) between identical twins (100% shared genes) and fraternal twins (50% shared genes). If identical twins have higher concordance, genetics play a role.
Overview of the Nervous System
The nervous system is your body's electrical wiring—a complex communication network that controls everything from thoughts to heartbeats.
| System | Components | Function |
|---|---|---|
| Central Nervous System (CNS) | Brain + Spinal Cord | Command center; processes information and makes decisions |
| Peripheral Nervous System (PNS) | All nerves outside CNS | Connects CNS to body; carries messages to and from the brain |
⚡ Sympathetic
"Fight or Flight"
Mobilizes body for action: increases heart rate, dilates pupils, inhibits digestion.
😌 Parasympathetic
"Rest and Digest"
Calms body: slows heart rate, constricts pupils, promotes digestion.
- Know the hierarchy: Nervous System → CNS/PNS → Somatic/Autonomic → Sympathetic/Parasympathetic.
- Sympathetic vs. Parasympathetic: They work in opposition—learn specific effects on organs.
- Reflex arc: Spinal cord can process some responses without brain involvement (faster reaction).
1) A student is about to give a speech and feels their heart racing. Which division of the autonomic nervous system is activated?
2) After the speech, the student relaxes. What system helps them calm down?
Show Answers
1) The sympathetic nervous system (fight-or-flight response) causes the racing heart in anticipation of the stressful event.
2) The parasympathetic nervous system (rest-and-digest) activates to return the body to its calm baseline state.
The Neuron and Neural Firing
Neurons are the building blocks of the nervous system—specialized cells that transmit information through electrical and chemical signals.
| Part | Function |
|---|---|
| Dendrites | Receive incoming signals from other neurons |
| Cell Body (Soma) | Contains nucleus; integrates incoming signals |
| Axon | Transmits electrical impulse away from cell body |
| Myelin Sheath | Fatty insulation that speeds up signal transmission |
| Terminal Buttons | Release neurotransmitters into the synapse |
- Resting Potential: Neuron is at -70mV (negative inside, positive outside).
- Stimulation: Dendrites receive excitatory signals.
- Threshold: If stimulation reaches -55mV, action potential fires.
- Depolarization: Sodium (Na⁺) rushes in → cell becomes positive.
- Repolarization: Potassium (K⁺) rushes out → cell returns to negative.
- Refractory Period: Brief rest before neuron can fire again.
Neurons don't touch—there's a tiny gap called the synapse. Communication happens chemically:
- Action potential reaches terminal buttons
- Vesicles release neurotransmitters into synaptic cleft
- Neurotransmitters bind to receptors on receiving neuron
- Excess neurotransmitters are cleared via reuptake or enzyme breakdown
| Neurotransmitter | Function | Related Conditions |
|---|---|---|
| Dopamine | Reward, motivation, movement | Parkinson's (too little), Schizophrenia (too much) |
| Serotonin | Mood, sleep, appetite | Depression (too little) |
| Acetylcholine (ACh) | Muscle movement, memory | Alzheimer's (too little) |
| GABA | Inhibitory; calms neural activity | Anxiety disorders (too little) |
| Glutamate | Excitatory; learning & memory | Seizures (too much) |
| Norepinephrine | Alertness, arousal | Depression, ADHD |
| Endorphins | Pain relief, pleasure | "Runner's high" |
1) What would happen if the myelin sheath were damaged?
2) An SSRI (antidepressant) blocks serotonin reuptake. Why would this help depression?
Show Answers
1) Signal transmission would slow down significantly. This is what happens in Multiple Sclerosis (MS), leading to muscle weakness and coordination problems.
2) Blocking reuptake keeps serotonin in the synapse longer, increasing its effects. Since depression is linked to low serotonin, more available serotonin can improve mood.
The Brain
The brain is the most complex organ in the known universe—approximately 86 billion neurons creating trillions of connections.
| Technique | What It Shows | Key Feature |
|---|---|---|
| EEG | Electrical activity (brain waves) | Excellent timing, poor location |
| CT Scan | Brain structure (X-ray images) | Detects tumors, damage |
| MRI | Detailed brain structure | High-resolution images |
| fMRI | Brain activity (blood flow) | Shows what areas are active |
| PET Scan | Brain activity (glucose use) | Uses radioactive tracers |
Location: Front of brain
Functions: Executive functions, planning, decision-making, personality, motor cortex (movement)
Famous case: Phineas Gage
Location: Top-back of brain
Functions: Somatosensory cortex (touch, temperature, pain), spatial awareness
Location: Sides of brain (near ears)
Functions: Auditory processing, language comprehension (Wernicke's area), memory
Location: Back of brain
Functions: Visual processing
Note: Damage causes blindness even with healthy eyes
| Structure | Function | Memory Tip |
|---|---|---|
| Thalamus | Sensory relay station (except smell) | "Thalamus = Telephone operator" |
| Hypothalamus | Homeostasis: hunger, thirst, temperature, circadian rhythms | "Hypo = 4 H's: Hungry, Horny, Hot/cold, Hours (sleep)" |
| Hippocampus | Memory formation (especially explicit/declarative) | "Hippo Campus = Memory campus" |
| Amygdala | Emotion (especially fear) and emotional memories | "Amygdala = Alarm system" |
| Cerebellum | Coordination, balance, procedural memory | "Little brain" at the back |
| Brainstem | Basic life functions (breathing, heart rate) | Medulla, pons, midbrain |
Left Hemisphere:
- Language (in most people)
- Logic and analysis
- Math calculations
- Controls right side of body
Right Hemisphere:
- Spatial abilities
- Face recognition
- Emotional processing
- Controls left side of body
The corpus callosum connects both hemispheres. Split-brain patients (severed corpus callosum) show dramatic lateralization effects.
1) A patient can understand speech but speaks in jumbled, meaningless sentences. Which area is likely damaged?
2) Which brain structure would be most active when you feel sudden fear?
Show Answers
1) Wernicke's area (temporal lobe). This describes Wernicke's aphasia—fluent but nonsensical speech with preserved comprehension.
2) The amygdala, which processes emotions, especially fear responses.
Sleep
Sleep isn't passive—it's an active process essential for memory consolidation, physical restoration, and cognitive function.
| Stage | Brain Waves | Characteristics |
|---|---|---|
| NREM Stage 1 | Theta waves | Light sleep; easily awakened; hypnagogic hallucinations |
| NREM Stage 2 | Sleep spindles, K-complexes | Body temperature drops; heart rate slows; ~50% of sleep |
| NREM Stage 3 | Delta waves (slow-wave) | Deep sleep; hard to wake; physical restoration; sleepwalking occurs |
| REM Sleep | Beta waves (like awake) | Rapid eye movement; vivid dreams; muscle paralysis; memory consolidation |
| Disorder | Description |
|---|---|
| Insomnia | Difficulty falling/staying asleep; most common sleep disorder |
| Narcolepsy | Sudden, uncontrollable sleep attacks; directly entering REM |
| Sleep Apnea | Breathing stops repeatedly during sleep; loud snoring; oxygen deprivation |
| Night Terrors | Occur in Stage 3 (NREM); no memory; more common in children |
| Sleepwalking | Occurs in Stage 3 (NREM); complex behaviors while asleep |
| REM Sleep Behavior Disorder | Acting out dreams due to lack of normal muscle paralysis |
- REM ≠ deep sleep. Stage 3 (slow-wave) is the deepest sleep.
- REM increases as the night progresses; Stage 3 decreases.
- Sleep deprivation effects: impaired memory, weakened immune system, increased accidents, mood problems.
1) Why are you unlikely to remember a night terror?
2) How does melatonin relate to the circadian rhythm?
Show Answers
1) Night terrors occur during Stage 3 NREM (deep sleep), when brain activity is low and memory consolidation isn't happening. Dreams that are remembered occur during REM.
2) Melatonin is a hormone released by the pineal gland in response to darkness. It makes you sleepy and helps regulate the circadian rhythm's sleep-wake cycle.
Sensation
Sensation is the process of detecting physical energy from the environment and converting it into neural signals. It's the first step in experiencing the world.
The difference threshold is a constant proportion of the original stimulus.
ΔI / I = k
Example: If you can just notice a 1 lb difference when holding 10 lbs, you'd need a 2 lb difference to notice a change when holding 20 lbs.
| Sense | Stimulus | Receptor | Brain Area |
|---|---|---|---|
| Vision | Light waves | Rods (dim light), Cones (color) | Occipital lobe |
| Hearing | Sound waves | Hair cells in cochlea | Temporal lobe |
| Touch | Pressure, temperature | Various skin receptors | Parietal lobe |
| Taste | Chemical molecules | Taste buds | Parietal lobe |
| Smell | Chemical molecules | Olfactory receptors | Directly to limbic system |
👁️ Vision
Rods: Peripheral vision, night vision, black & white
Cones: Center of retina (fovea), color vision, detail
Blind spot: Where optic nerve exits (no receptors)
👂 Hearing
Pitch: Determined by frequency (Hz)
Loudness: Determined by amplitude (dB)
Conduction deafness: Mechanical problem
Sensorineural deafness: Nerve damage
- Smell is unique: Only sense that bypasses the thalamus; goes directly to limbic system (emotional memories).
- Gate-control theory: Spinal cord has a "gate" that can block or allow pain signals to the brain.
- Know the pathway: Physical stimulus → Receptor → Transduction → Neural signal → Brain
Trichromatic Theory: Three types of cones (red, green, blue) combine to create all colors we see. Explains color blindness.
Opponent-Process Theory: Color perception involves opponent pairs (red-green, blue-yellow, black-white). Explains afterimages.
Both theories are correct! Trichromatic works at the receptor level; opponent-process works at the neural pathway level.
1) Why does smell trigger such strong memories?
2) According to Weber's Law, if you can just notice a difference between 100g and 102g, what would be the JND for 200g?
Show Answers
1) Smell is the only sense that bypasses the thalamus and goes directly to the limbic system (including the hippocampus and amygdala), which are involved in emotion and memory.
2) The Weber fraction is 2/100 = 0.02 (2%). For 200g: 200 × 0.02 = 4g. You'd need a 4g difference to notice a change.