3.4 Genetics, Epigenetics, Endocrine Systems, and Health Behavior

Biological vulnerability is probabilistic, not destiny

Genetics appears under biological bases of behavior, but the EPPP rewards nuance. Heritability (h-squared) is the proportion of variation in a trait within a population associated with genetic differences under particular environmental conditions. It ranges from 0 to 1 and describes a population, not an individual. A heritability of 0.5 does not mean half of one person's depression is genetic; it means about half the variability across that group, in that setting, tracks genetic variation.

Most psychiatric conditions are polygenic (many small-effect genes) rather than single-gene. Twin and adoption studies estimate heritability: concordance is higher in monozygotic (identical) than dizygotic (fraternal) twins for schizophrenia, bipolar disorder, and autism, yet concordance is well below 100%, proving environment matters. Schizophrenia, for instance, shows roughly 50% MZ concordance — high vulnerability, not certainty.

Gene-environment interaction means an environment's effect depends on genotype (and vice versa) — a classic example is how early adversity raises risk more strongly in people with certain genetic profiles. Gene-environment correlation means inherited tendencies shape the environments people select, evoke, or are given (passive, evocative, active rGE). Epigenetics, at exam level, means experience can change gene expression through mechanisms such as DNA methylation and histone modification, without altering the underlying sequence.

Endocrine systems bridge body and mind

The endocrine system connects physiology and psychology. Thyroid dysfunction is high-yield: hypothyroidism mimics depression (fatigue, weight gain, cold intolerance, slowed cognition), while hyperthyroidism mimics anxiety (restlessness, palpitations, weight loss). Cortisol, released through the HPA axis, governs the stress response; chronically elevated cortisol affects sleep, memory, and immunity. Sex hormones interact with mood across life-stage transitions (premenstrual, postpartum, perimenopausal). Glucose dysregulation, inflammation, chronic pain, and sleep disorders all shape mental-health presentation.

Health behavior

Health psychology bridges biology and behavior. Exercise, nutrition, sleep, substance use, medication adherence, pain behavior, and medical follow-through all influence functioning. Useful frameworks include the Transtheoretical (stages of change) model (precontemplation, contemplation, preparation, action, maintenance), the Health Belief Model (perceived susceptibility, severity, benefits, barriers), self-efficacy, social support, and reinforcement.

Scenario pattern. A client reports low mood, weight gain, fatigue, poor concentration, and cold intolerance. A strong answer considers major depression and hypothyroidism, and arranges medical referral before locking into a purely psychological diagnosis. Scenario pattern. A stem gives a family history of bipolar disorder. The correct inference is increased vulnerability, not certainty; the clinician still assesses mood episodes, sleep, substances, medications, impairment, and course.

Practical study list:

• Distinguish genetic influence from genetic determinism.
• Connect endocrine changes (thyroid, cortisol) to symptoms that mimic psychological disorders.
• Treat sleep and pain as biological variables affecting attention, mood, and treatment response.
• Use family history as one data point, not a diagnosis.
• Remember protective environments reduce risk and support resilience.

The diathesis-stress and biopsychosocial frames

Two integrative models tie this section together. The diathesis-stress model holds that a predisposition (genetic, biological, or psychological vulnerability) interacts with environmental stress to produce a disorder; neither alone is usually sufficient. This explains why identical twins are discordant for schizophrenia and why family history raises but does not guarantee risk. The biopsychosocial model (Engel) insists that biological, psychological, and social factors jointly shape health and illness, and that intervention can enter at any level.

On the EPPP, an answer that names a single biological cause for a clearly multi-determined presentation is usually a distractor; the keyed answer integrates levels.

Immune and inflammatory links

The exam increasingly recognizes connections among stress, inflammation, and mood. Chronic stress and elevated cortisol can suppress and dysregulate immune function, and inflammatory processes are associated with fatigue, low mood, and "sickness behavior." Sleep loss compounds these effects. Practically, this means a psychologist should treat sleep, pain, nutrition, and medical illness as legitimate contributors to a mood or anxiety presentation rather than nuisance variables.

From risk to resilience

Protective factors — secure relationships, stable environments, coping skills, social support, and access to care — buffer biological vulnerability. The same family-history stem that signals risk also invites attention to strengths. This material should make candidates more cautious, not more reductionistic. Biological explanations are strongest when integrated with development, assessment, culture, behavior, and context — and when they respect the limits of what a case stem can prove. A licensure-level answer holds vulnerability and agency together, treating biology as one input among several rather than a verdict.

Watch for two recurring distractor traps in this domain. The first equates a high heritability estimate with a fixed prognosis, ignoring that heritability is conditional on environment and says nothing about treatability. The second treats a single biomarker, hormone level, or family-history fact as diagnostic, when the keyed answer gathers a fuller history and considers medical referral. Whenever a stem hands you a neat biological cause, ask whether the presentation is actually multi-determined; on the EPPP, the integrative, biopsychosocial response is usually the one the item writers reward.