Individual influences on behavior

PsychBio

The factors inside the individual that shape behavior: the biology (brain, neurotransmitters, hormones, genes), personality, psychological disorders, and motivation.

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Biological bases of behavior

BioPsych high-yield

The nervous system, key neurotransmitters, the endocrine system, brain regions, how we study them, and the genetics of behavior.

The nervous system splits into central (brain + spinal cord) and peripheral; the peripheral into somatic (voluntary) and autonomic, which itself splits into sympathetic ("fight or flight") and parasympathetic ("rest and digest"). Key neurotransmitters and behavioral roles: dopamine (reward, movement; ↑ in schizophrenia, ↓ in Parkinson's), serotonin (mood, sleep, appetite; low in depression), acetylcholine (muscle action, memory; low in Alzheimer's), GABA (main inhibitory), glutamate (main excitatory), norepinephrine (arousal, alertness), and endorphins (pain relief). The endocrine system uses hormones (e.g., cortisol for stress, adrenaline/epinephrine for arousal) — slower but longer-lasting than neural signaling. Brain regions: hindbrain (medulla, pons, cerebellum — vital functions, balance), midbrain (arousal, sensorimotor reflexes), forebrain (thalamus, hypothalamus, limbic system, cerebral cortex), with the four cortical lobes (frontal — executive/movement; parietal — sensation; temporal — hearing/language/memory; occipital — vision). Study methods: EEG, fMRI/PET, and lesion studies. Behavioral genetics (twin and adoption studies, heritability) addresses the nature vs. nurture question — most traits are both, via gene–environment interaction.

Labeled diagram of a multipolar neuron showing dendrites, cell body (soma), axon hillock, axon, myelin sheath, nodes of Ranvier, Schwann cell, and axon terminals, with the signal traveling from dendrites to terminals, plus an inset of a synapse showing synaptic vesicles, the synaptic cleft, neurotransmitters, and postsynaptic receptors. — A neuron receives input at its dendrites, integrates it in the soma, and fires an action potential down the myelinated axon (jumping between nodes of Ranvier) to the axon terminals, where neurotransmitters cross the synaptic cleft to receptors on the next cell.

Lateral view of the cerebral hemisphere with the four lobes color-coded (frontal, parietal, temporal, occipital), plus the cerebellum and brainstem, and the functional areas labeled: primary motor cortex and primary somatosensory cortex on either side of the central sulcus, Broca's area in the inferior frontal lobe, Wernicke's area in the posterior temporal lobe, primary visual cortex in the occipital lobe, and primary auditory cortex in the superior temporal lobe. — The four cortical lobes and their key functional areas: motor (precentral) and somatosensory (postcentral) cortices flank the central sulcus; Broca's area (frontal) drives speech production and Wernicke's area (temporal) speech comprehension; vision maps to the occipital lobe, hearing to the temporal lobe.

Midsagittal section of the brain labeling the cerebral cortex, corpus callosum, thalamus, hypothalamus, pituitary gland, pineal gland, and the brainstem divided into midbrain, pons, and medulla oblongata, with the cerebellum at the back. — A midline view of the brain's deep structures: the thalamus (sensory relay) and hypothalamus (homeostasis, with the pituitary gland below it) sit above the brainstem — midbrain, pons, and medulla oblongata — with the cerebellum behind.

Medial view of the brain highlighting the limbic system: cingulate gyrus, fornix, thalamus, hypothalamus, amygdala, hippocampus, and olfactory bulb, with the amygdala at the anterior tip of the hippocampus. — The limbic system governs emotion and memory: the amygdala (fear and threat detection) sits at the front of the hippocampus (memory formation), with the hypothalamus, thalamus, cingulate gyrus, and fornix completing the circuit.

Comparison diagram of the autonomic nervous system: the sympathetic division (fight or flight) on the left and the parasympathetic division (rest and digest) on the right, showing their opposing effects on the eyes, salivary glands, heart, lungs, stomach and intestines, adrenal gland, and bladder. — The two autonomic divisions act in opposition: the sympathetic ('fight or flight') dilates the pupils, speeds the heart, and relaxes the bladder; the parasympathetic ('rest and digest') constricts the pupils, slows the heart, and stimulates digestion.

Diagram of the endocrine system showing the major glands on a neutral body silhouette — hypothalamus, pituitary, pineal, thyroid, parathyroid glands, thymus, adrenal glands, and pancreas — with the gonads in separate insets: ovaries (female) and testes (male). — The endocrine system's major hormone-secreting glands: the hypothalamus and pituitary direct the system, with the thyroid, parathyroids, thymus, adrenal glands, pancreas, and gonads (ovaries and testes) regulating metabolism, stress, and reproduction.

Personality

Psych high-yield

Five major perspectives explain consistent patterns of thought, feeling, and behavior.

Psychoanalytic / psychodynamic (Freud) — behavior is driven by the unconscious. The id (pleasure principle), ego (reality principle), and superego (morality), with defense mechanisms (repression, denial, projection, displacement, sublimation, regression, rationalization, reaction formation) managing conflict. Neo-Freudians: Jung (collective unconscious, archetypes), Adler (inferiority complex), Horney.
Humanistic — emphasizes growth and free will. Maslow (self-actualization) and Rogers (unconditional positive regard, the self-concept, congruence).
Trait — personality as stable traits. The Big Five (OCEAN): Openness, Conscientiousness, Extraversion, Agreeableness, Neuroticism. Also Eysenck's dimensions and the person–situation (consistency) debate.
Social-cognitive (Bandura) — reciprocal determinism (person × behavior × environment) and locus of control (internal vs. external).
Biological / trait-temperament — genetic and physiological bases of temperament.

How AAMC tests it

Match a described behavior or quote to the correct perspective, or identify a specific defense mechanism from a scenario.

Psychological disorders

PsychBio med-yield

Major categories of disorder and the competing models of what causes them.

The biomedical model views disorders as purely biological; the biopsychosocial model integrates biological, psychological, and social factors — and pairs with the diathesis-stress model (a predisposition + a stressor triggers the disorder). High-yield categories: anxiety disorders (generalized, panic, phobias), obsessive-compulsive, depressive and bipolar disorders, schizophrenia (positive symptoms like hallucinations/delusions vs. negative symptoms like flat affect; the dopamine hypothesis), dissociative disorders, somatic symptom disorders, and personality disorders (e.g., antisocial, borderline). Be able to connect symptoms to neurotransmitter imbalances (e.g., low serotonin/depression, high dopamine/schizophrenia).

Motivation

Psych high-yield

Why we act: theories from biological drives to a hierarchy of needs.

Instinct theory — behavior is driven by innate, fixed patterns.
Drive-reduction theory — physiological needs create drives that motivate us to restore homeostasis (primary drives like hunger; secondary/learned drives).
Arousal theory — we seek an optimal level of arousal; the Yerkes-Dodson law says performance peaks at moderate arousal (and the optimum is lower for hard tasks).
Incentive theory — behavior is pulled by external rewards/incentives.
Maslow's hierarchy of needs — physiological → safety → love/belonging → esteem → self-actualization; lower needs are generally met first.

Also: intrinsic vs. extrinsic motivation and the overjustification effect (external reward can undercut intrinsic motivation). Specific motivators include hunger (the hypothalamus, leptin/ghrelin) and sexual motivation.

Don't confuse

Drive-reduction (push from an internal need) vs. incentive (pull from an external reward). And note arousal theory's inverted-U (Yerkes-Dodson).

Worked question

A boy who used to draw for fun is given $5 for every picture he completes. Weeks later, after the payments stop, he draws far less than he did before any reward existed. This is best explained by: