Influences on Behaviour

Behavior is shaped by biology at several different levels — the fast, moment-to-moment chemical signals neurons send each other, the slower, body-wide hormone signals the endocrine system releases, and the genetic blueprint inherited from our parents. This subtopic works through all three: the neurotransmitters that carry signals across the synapse, the glands and hormones that make up the endocrine system, and the family, twin, and adoption studies researchers use to untangle how much of behavior comes from nature versus nurture.

Key Takeaways

• Neurons communicate across the synapse using neurotransmitters, which either excite or inhibit the next neuron; agonists increase signaling along a pathway, antagonists decrease it, and medications frequently work through one mechanism or the other.

• Epinephrine, norepinephrine, and dopamine (the catecholamines, synthesized from tyrosine), serotonin, GABA, glutamate, acetylcholine, and endorphins each have distinct, MCAT-relevant roles in emotion, cognition, and movement.

• The endocrine system communicates more slowly but more broadly than the nervous system, using hormones released by glands — the hypothalamus, pituitary, adrenal glands, thyroid, parathyroid glands, pancreas, and gonads — to regulate growth, metabolism, stress, and reproduction.

• Behavior arises from the ongoing interaction of genetics and environment, not one or the other; family, twin, and adoption studies are the three main research designs used to estimate how much a given trait is shaped by genetic versus environmental factors, each controlling for a different confound.

How Neurons Communicate: Neurotransmitters

Neurons aren't physically connected to one another. Between any two communicating neurons sits a tiny gap called the synapse, and the way information crosses that gap is through neurotransmitters — chemical messengers released by an activated neuron that travel across the synapse and bind to receptors on the next cell.

Not every neurotransmitter has the same effect once it binds. Some make the next neuron more likely to fire, continuing the signal; others make it less likely to fire, dampening the signal. The nervous system is constantly balancing these excitatory and inhibitory signals against each other, and that balance helps regulate everything from movement and sensation to emotion and behavior.

Agonists and Antagonists

Two terms come up constantly once medications and drugs enter the picture: agonist and antagonist.

An agonist increases signaling along a particular pathway — either by activating a receptor directly or by mimicking the neurotransmitter that normally binds there. An antagonist does the opposite: it blocks the receptor, preventing the neurotransmitter from producing its usual effect and reducing signaling along that pathway. Medications work the same way — some act as agonists and strengthen a given pathway's signaling, while others act as antagonists and suppress it.

The Major Neurotransmitters

A handful of neurotransmitters come up again and again on the MCAT. Grouping them by shared properties makes them easier to keep straight.

Catecholamines: Epinephrine, Norepinephrine, and Dopamine

Epinephrine (adrenaline) is one of the chemicals that drives the fight-or-flight response. When epinephrine levels rise, heart rate increases, blood pressure rises, more glucose becomes available in the bloodstream, and overall alertness increases as the body shifts into a more activated state.

Norepinephrine (noradrenaline) is closely related to epinephrine and helps regulate alertness, attention, and arousal — it's active whenever something suddenly grabs your attention or you're deeply focused on a task. Outside the brain, norepinephrine also contributes to sympathetic nervous system activity, constricting blood vessels and helping regulate blood pressure, and together with epinephrine it helps prepare the body for action.

Dopamine is best known for its role in motivation and reinforcement learning: when a behavior leads to a rewarding or important outcome, dopamine helps reinforce that behavior and makes it more likely to be repeated. It's often oversimplified as "the pleasure neurotransmitter," but a more accurate way to think about it is that dopamine helps the brain decide what's worth pursuing — it assigns significance to experiences and encourages the repetition of behaviors the brain judges valuable. Dopamine also plays a major role in movement, which is why the progressive loss of dopamine-producing neurons is central to Parkinson's disease.

Epinephrine, norepinephrine, and dopamine together belong to a group called catecholamines, all synthesized from the amino acid tyrosine.

MCAT Callout — Catecholamine Synthesis: Catecholamine synthesis follows one pathway: tyrosine → L-DOPA → dopamine → norepinephrine → epinephrine. Tyrosine hydroxylase converts tyrosine to L-DOPA in the rate-limiting step. This is why dopamine, norepinephrine, and epinephrine are grouped together as catecholamines — each is built from the last.

Serotonin

Serotonin is best known for its role in mood and emotional regulation, but its effects extend further — it also contributes to sleep, appetite, and digestion. Because of its role in mood regulation, serotonin comes up frequently in discussions of depression and anxiety, and many antidepressant medications work by increasing serotonin signaling in the brain.

GABA and Glutamate

GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in the central nervous system — when GABA binds its receptors, it generally makes neurons less likely to fire, acting as one of the brain's natural ways of slowing activity down. That inhibition matters: without enough of it, neural circuits can become overactive and unstable, and in severe cases that excess activity can contribute to seizures. Many anti-anxiety medications and sedatives work by enhancing GABA signaling, increasing inhibition and producing a calming effect.

Glutamate does the opposite job. It's the primary excitatory neurotransmitter in the central nervous system, making neurons more likely to become active rather than less. Glutamate is essential for learning and memory because it drives synaptic plasticity — the brain's ability to strengthen or weaken connections between neurons over time as a result of experience. But excess is a problem here too: when glutamate signaling becomes excessive, neurons can become overstimulated, and that overstimulation can eventually damage them.

MCAT Callout — GABA vs. Glutamate: GABA and glutamate are easy to mix up because their names sound similar and their jobs are opposite. GABA = inhibitory (slows neurons down). Glutamate = excitatory (speeds neurons up). The two work as a balancing pair — synaptic plasticity depends on glutamate's excitatory push, while GABA keeps that excitation from spiraling out of control.

Acetylcholine

Acetylcholine is involved in communication between nerves and skeletal muscles — at the neuromuscular junction, it triggers muscle contraction. Within the brain, acetylcholine also contributes to attention, learning, and memory.

MCAT Callout — Cholinergic Hypothesis of Alzheimer's: Just as dopamine loss is linked to Parkinson's disease, acetylcholine loss is linked to Alzheimer's disease. The cholinergic hypothesis of Alzheimer's disease points to the degeneration of acetylcholine-producing neurons — particularly in a region called the nucleus basalis of Meynert — as a major contributor to the memory and learning decline seen in the disorder.

Endorphins

Endorphins are natural opioid-like molecules the body produces on its own. Their best-known function is reducing pain perception, and they can also improve mood. Endorphin release typically increases during intense or prolonged exercise, which is part of why people sometimes describe a "runner's high" — reduced pain perception paired with an improved mood during and after strenuous activity.

MCAT Callout — Runner's High: The classic explanation for runner's high is "endorphins cause it," but the full picture is more nuanced. Endorphins reliably reduce pain perception during exercise, but circulating endorphins in the blood don't cross the blood-brain barrier easily, which makes it hard for them alone to fully explain a euphoric feeling generated inside the brain. Current research points to the endocannabinoid system as a major, possibly leading, contributor to that specific euphoric sensation, likely working alongside endorphins rather than in place of them. Know endorphins for pain reduction; treat the "cause of the euphoria" question as an active area of research rather than a settled fact.

The Endocrine System

Neurons aren't the body's only communication system. The endocrine system works alongside the nervous system, but the two communicate in very different ways. Neurons send rapid signals across synapses using electrical and chemical signaling. Hormones take a different route entirely — they're released into the bloodstream and travel throughout the body until they reach their target tissues. Because of that, endocrine signaling is generally slower than nervous system signaling, but hormonal effects tend to be more widespread and longer-lasting, making the endocrine system well suited to regulating processes that need to be coordinated over longer stretches of time.

The endocrine system consists of specialized structures called glands, which release hormones that regulate processes throughout the body — growth, metabolism, reproduction, stress responses, fluid balance, and energy regulation among them.

The Hypothalamus and Pituitary Gland

The hypothalamus is one of the major links between the nervous system and the endocrine system: it constantly receives information about the body's internal state and communicates with the pituitary gland to help coordinate endocrine activity throughout the body (the mechanics of exactly how the hypothalamus reaches each part of the pituitary are covered in more depth in the forebrain-focused subtopic of this chapter).

The pituitary gland has two major divisions. The anterior pituitary produces and releases its own hormones. The posterior pituitary works differently — rather than producing its own hormones, it stores and releases hormones that were originally produced by neurons in the hypothalamus. Two of those hormones are worth knowing by name: antidiuretic hormone (ADH), which helps regulate water balance, and oxytocin, which is involved in processes such as childbirth, breastfeeding, and social bonding.

The Adrenal Gland

The adrenal gland sits on top of each kidney and contains two functionally distinct regions.

The adrenal medulla is associated with the body's immediate stress response. During stressful situations, it releases epinephrine and norepinephrine into the bloodstream — the same molecules discussed earlier as neurotransmitters, now acting as hormones. Once in circulation, they help produce many of the same fight-or-flight effects covered above: increased heart rate, increased alertness, and more energy made available to the body's tissues.

The adrenal cortex produces different hormones for a different timescale. Cortisol is associated with the body's longer-term response to stress and helps regulate metabolism and energy use. Aldosterone helps regulate blood pressure and fluid balance by influencing how the kidneys retain sodium and water.

MCAT Callout — Adrenal Medulla vs. Adrenal Cortex: Same gland, two different jobs. Adrenal medulla = epinephrine/norepinephrine, fast fight-or-flight response. Adrenal cortex = cortisol and aldosterone, slower, longer-lasting stress and fluid regulation.

Thyroid, Parathyroid, and Pancreas

The thyroid gland helps regulate metabolism throughout the body — its hormones influence how quickly cells use energy, which is why changes in thyroid function can affect energy levels and overall metabolic rate.

Located just behind the thyroid, the parathyroid glands help regulate calcium levels in the body, an important job since calcium is involved in muscle contraction and bone physiology.

The pancreas produces two hormones that work together to keep blood glucose within a healthy range: insulin, which lowers blood glucose by promoting glucose uptake into cells when levels get too high, and glucagon, which raises blood glucose when levels start to drop.

Gonads

The gonads — ovaries in females, testes in males — produce sex hormones involved in reproduction and development, including the physical changes that occur during puberty.

Nature vs. Nurture: Genetics and Behavior

Just as physical traits can be inherited, behavioral tendencies and predispositions can be influenced by genetic factors, which raises a classic question: how much of our behavior is shaped by our genes, and how much by our environment? This is the nature versus nurture question — nature referring to biology, including genes, and nurture referring to the experiences and environments we grow up in and live through.

For a long time, this was treated as an either-or question. Today, researchers understand it differently: biology and environment are constantly interacting rather than competing. Genes influence how a person responds to the world, and experiences in turn shape the brain and change how it functions. Behavior is best understood as the product of both working together, not one or the other.

The challenge is figuring out how much of a given behavior traces to genetics versus environment, and researchers use three main approaches — family studies, twin studies, and adoption studies — to look for patterns. None of these methods asks whether a behavior is caused entirely by genes or entirely by environment; instead, they ask whether people who share more genes also tend to be more similar in the trait or behavior being studied.

Family Studies

A family study looks at whether a particular trait or disorder tends to run in families. If a condition shows up more often among someone's biological relatives than in the general population, that pattern suggests genetics may be contributing. For example, researchers might look at whether schizophrenia occurs more frequently among the biological relatives of someone who has schizophrenia — a consistent pattern like that is evidence that inherited genetic factors could be involved.

The catch is that family members don't just share genes — they typically share a home, an environment, and many of the same life experiences. If two siblings develop the same disorder, it's hard to know how much of that similarity comes from shared genetics and how much comes from growing up in the same environment. That limitation is exactly why researchers developed twin and adoption studies as complementary approaches.

Twin Studies

A twin study compares identical twins and fraternal twins to see whether greater genetic similarity is associated with greater similarity in a given trait or disorder.

Identical (monozygotic) twins develop from a single fertilized egg that later splits into two embryos, so they share nearly all of their genetic material. Fraternal (dizygotic) twins develop from two separate fertilized eggs and, on average, share about half their genes — roughly the same proportion shared by any pair of siblings.

Researchers compare something called the concordance rate — the probability that both twins in a pair show the same trait or disorder. Using schizophrenia again as an example: if identical twins consistently show a much higher concordance rate than fraternal twins, that pattern suggests genetics plays an important role, since the group sharing more DNA also shows greater similarity in the outcome. That doesn't mean genes fully determine the outcome — only that genetic factors appear to contribute to the risk.

Twin studies become especially informative when identical twins are raised apart. Those twins share nearly identical genetics while growing up in different environments, so striking similarities later in life provide stronger evidence for a genetic contribution — though ongoing differences in their separate environments still have to be considered, since experience continues shaping behavior throughout life.

MCAT Callout — Concordance Rate vs. Heritability: Concordance rate ≠ heritability. Concordance rate is simply the probability both twins in a pair share a trait. A higher concordance rate in monozygotic twins than dizygotic twins is evidence genetics contributes to a trait — it isn't a precise measurement of exactly how much.

Adoption Studies

An adoption study compares adopted individuals with both their biological relatives and their adoptive relatives. This design separates genetics from environment more cleanly than a family study can, because an adopted child shares genes with their biological family but grows up in the environment provided by their adoptive family. That setup lets researchers ask which family the child resembles more: a trait that's much more common among biological relatives than adoptive relatives points toward genetics, while a trait that more closely tracks the adoptive family points toward environmental influence.

Adoption studies aren't perfect either — biological and environmental factors can interact even before birth, and early-life experiences continue shaping development afterward. Like family and twin studies, adoption studies provide valuable insight without offering a perfectly clean separation between nature and nurture.

Why Influences on Behaviour Matter for the MCAT

This subtopic sits at the intersection of biology and behavior, a recurring theme across MCAT Behavioral Sciences. Watch for:

• GABA vs. glutamate. Inhibitory vs. excitatory — opposite jobs, similar-sounding names.

• Agonist vs. antagonist. Increases signaling vs. blocks/decreases signaling along a pathway.

• Adrenal medulla vs. adrenal cortex. Fast fight-or-flight hormones (epinephrine/norepinephrine) vs. slower stress/fluid regulation hormones (cortisol/aldosterone).

• Dopamine as more than "the pleasure neurotransmitter." Motivation, reinforcement, and significance-assignment, plus a major role in movement.

• Concordance rate logic in twin studies. Higher concordance in monozygotic vs. dizygotic twins is evidence for genetic contribution, not proof of a fixed genetic percentage.

• Family vs. twin vs. adoption studies. Each design controls for a different confound — twin studies isolate genetic similarity, adoption studies isolate shared environment.

Common MCAT Mistakes

• Mixing up GABA and glutamate. Their names sound alike, but their jobs are opposite: GABA is inhibitory (slows neurons down), glutamate is excitatory (speeds neurons up). Don't let the similar spelling override the opposite function.

• Reducing dopamine to "the pleasure chemical." Dopamine's core MCAT role is motivation, reinforcement, and assigning significance to experiences — plus a major role in movement (relevant to Parkinson's disease) — not simply generating pleasure.

• Confusing the adrenal medulla with the adrenal cortex. The medulla releases epinephrine/norepinephrine for the immediate stress response; the cortex releases cortisol/aldosterone for longer-term stress and fluid regulation. Same gland, different hormones, different timescales.

• Treating a higher concordance rate as a precise genetic percentage. A higher concordance rate in monozygotic vs. dizygotic twins is evidence that genetics contributes to a trait — it doesn't quantify exactly how much of the trait is genetic.

MCAT-Style Concept Check

Question: A study finds a schizophrenia concordance rate of 48% among monozygotic twins and 17% among dizygotic twins. Which conclusion is best supported by this finding?

• A) Genetic factors do not meaningfully contribute to schizophrenia risk, since even identical twins fail to show perfect concordance.

• B) Genetic factors likely contribute to schizophrenia risk, but environmental factors also play a substantial role.

• C) Environmental factors play no role in schizophrenia, since monozygotic twins share nearly all their genes.

• D) The concordance rate directly measures the percentage of schizophrenia risk attributable to genetics.

Answer: B

Explanation: The much higher concordance rate in monozygotic twins (who share nearly all their genes) compared to dizygotic twins (who share about half) suggests genetic factors contribute to schizophrenia risk — the group sharing more DNA also shows greater similarity in the outcome. But monozygotic concordance is far below 100%, so genetics alone doesn't determine the outcome — environmental factors must also play a substantial role. This rules out (A), which dismisses genetics despite the large MZ/DZ gap, and (C), which dismisses environment despite imperfect MZ concordance. (D) is incorrect because concordance rate is a probability that both twins share a trait, not a precise measurement of heritability.

FAQ

What's the difference between an agonist and an antagonist?

An agonist increases signaling along a pathway, either by activating a receptor directly or mimicking the neurotransmitter that normally binds it. An antagonist blocks the receptor and reduces signaling along that pathway. Many medications work through one mechanism or the other.

Why are epinephrine, norepinephrine, and dopamine grouped together as catecholamines?

All three are synthesized from the amino acid tyrosine along the same pathway: tyrosine → L-DOPA → dopamine → norepinephrine → epinephrine. Because each is built from the last step in that shared pathway, they're classified together as catecholamines.

What's the difference between the adrenal medulla and the adrenal cortex?

The adrenal medulla releases epinephrine and norepinephrine for the body's immediate, fast fight-or-flight response. The adrenal cortex releases cortisol and aldosterone, which handle longer-term stress response and fluid/blood pressure regulation. Same gland, two different regions with two different jobs.

How do twin studies help separate genetic and environmental influences on behavior?

Twin studies compare concordance rates between monozygotic twins (who share nearly all their genes) and dizygotic twins (who share about half). A higher concordance rate in monozygotic twins suggests genetics contributes to a trait, since the group sharing more DNA shows greater similarity in the outcome — though it doesn't precisely quantify how much of the trait is genetic.