Genes and Behaviour

students, have you ever wondered why some people seem naturally calm under stress, while others are more likely to be impulsive or anxious? 🧠 Or why some talents, like musical ability or athletic build, seem to run in families? In this lesson, you will explore how genes influence behaviour and how psychologists study that influence in scientifically careful ways.

Learning goals

By the end of this lesson, students, you should be able to:

• explain key ideas and terms related to genes and behaviour
• describe how genes and environment work together
• use IB Psychology reasoning to interpret studies on genetic influences
• connect genes and behaviour to the biological approach to understanding behaviour
• support ideas with examples from research and real life

Genes and behaviour is a major part of the biological approach because it asks a central question: how much of what we do is influenced by inheritance, and how much is shaped by experience? The answer is not simple. Behaviour is usually the result of both biology and environment working together.

What are genes and why do they matter?

Genes are sections of DNA that contain instructions for making proteins. These proteins help build and maintain the body, including the brain and nervous system. Since the brain controls thoughts, emotions, and actions, genes can influence behaviour indirectly by affecting brain structure, brain chemistry, and body development.

Humans have about 20,000 protein-coding genes. Each person inherits one copy of each gene from each biological parent, so most genes come in pairs. Different versions of the same gene are called alleles. For example, one allele may slightly increase the chance of a trait, while another may reduce it.

It is important to understand that genes do not usually cause a behaviour in a simple one-gene, one-behaviour way. Most psychological traits, such as intelligence, aggression, depression risk, or extraversion, are polygenic, meaning they are influenced by many genes working together. This makes behaviour complex and difficult to predict from a single gene alone.

A useful idea in IB Psychology is that genes provide a predisposition, not a fixed destiny. A predisposition is a tendency or likelihood, not a guarantee. For example, a person may inherit genes linked to high anxiety sensitivity, but whether that person actually develops an anxiety disorder may depend on stress, parenting, trauma, sleep, and social support.

How genes influence behaviour

Genes influence behaviour through biological pathways. One important pathway is the brain. Genes help shape how neurons form, how they connect, and how they communicate. They also influence neurotransmitters, which are chemical messengers in the brain. If gene differences affect dopamine, serotonin, or other systems, this can influence mood, motivation, attention, or impulse control.

Genes can also influence physical traits that affect behaviour. For example, genes may affect body size, reaction speed, or hormone levels. Hormones such as cortisol and testosterone are not behaviours themselves, but they can influence how people respond to stress, competition, or threat.

Another key concept is gene expression. A gene may be present, but it is not always active in the same way. Environmental factors can increase or decrease gene expression. This means that experience can affect how strongly certain genes influence the body and behaviour. This is one reason why nature and nurture are linked rather than separate.

A famous real-world example is the way stress can affect gene expression in people who experience early adversity. Research in biological psychology shows that supportive environments may reduce the harmful effects of genetic risk, while chronic stress may increase those effects. This helps explain why two people with similar genetic risk can have very different outcomes.

Behaviour is influenced by both genes and environment

The biological approach does not claim that genes alone determine behaviour. Instead, it examines the interaction between heredity and environment. This interaction is often called gene-environment interaction.

A gene-environment interaction happens when the effect of a gene depends on the environment. For instance, a person might carry a genetic risk for depression, but only develop symptoms after major life stress. Another person with the same genetic risk might never develop depression if their environment is protective.

A related idea is gene-environment correlation. This happens when a person’s genes influence the kind of environment they experience. For example, a child who is naturally energetic and curious may seek out more stimulating activities, which then provide more opportunities to develop certain skills. In this way, genes can shape behaviour indirectly by shaping experience.

This is why psychologists avoid saying a behaviour is “100% genetic” or “100% environmental.” Such statements are too simple. Instead, biological psychologists investigate how genes, brains, hormones, and life experiences combine to shape behaviour.

How psychologists study genes and behaviour

Because genes cannot be directly “seen” as behaviour, psychologists use scientific methods to study patterns of inheritance and risk. Three common approaches are twin studies, adoption studies, and molecular genetics.

Twin studies compare identical twins, who share nearly all their genes, with fraternal twins, who share about half their genes on average. If identical twins are more similar than fraternal twins for a trait, that suggests a genetic influence. This is called concordance, meaning the degree to which both twins show the same trait.

For example, if identical twins are more likely than fraternal twins to both have schizophrenia, researchers may conclude that genes play a role. However, twin studies do not prove that genes are the only cause. Identical twins often grow up in more similar environments than fraternal twins, so environment still matters.

Adoption studies help separate genetic and environmental effects. If a child resembles their biological parents more than their adoptive parents for a trait, that suggests a genetic influence. If the child resembles adoptive parents more, that suggests environmental influence. Adoption studies are useful, but they are also limited because adopted children may not represent the general population.

Molecular genetics looks directly at DNA to identify specific genes or genetic patterns associated with behaviour. Researchers may compare people with and without a disorder or use genome-wide association studies to search for links across the genome. These studies have shown that many behaviours and disorders are influenced by many small genetic effects rather than one single gene.

A well-known example is the $5-HTTLPR$ gene region, which has been studied in relation to depression risk and stress. Research suggests that certain gene variants may increase vulnerability to depression especially when combined with stressful life events. This is a strong example of gene-environment interaction.

Strengths and limitations of genetic explanations

Genetic explanations are valuable because they are scientific, testable, and often based on large data sets. They help explain why some traits run in families and why people differ in vulnerability to disorders. This knowledge can also guide treatment and prevention. For example, if a person is at higher genetic risk for a disorder, early support may reduce the chance of severe problems later.

However, there are important limitations. First, most psychological traits are polygenic, so it is difficult to identify a single cause. Second, genetic research often focuses on correlations, not direct causes. A correlation shows an association, but it does not prove that a gene causes a behaviour.

Third, many studies depend on self-report or family history, which can be inaccurate. Fourth, twin studies rely on assumptions, such as the idea that identical and fraternal twins experience equally similar environments. If this assumption is not fully true, the estimate of genetic influence may be distorted.

There are also ethical issues. Genetic research can lead to stigma if people think a disorder is “in the genes” and therefore hopeless. This is not accurate. Genes may increase risk, but environment and treatment still matter greatly. Psychologists must communicate genetic findings carefully so that people understand that risk is not destiny.

Genes and behaviour in the broader biological approach

Genes and behaviour fit within the biological approach because this approach explains behaviour using physical and chemical processes in the body. The biological approach includes the brain, the nervous system, hormones, genetics, and evolution. Genes are one part of this wider system.

In IB Psychology HL, you should see genes and behaviour as connected to other biological topics, not isolated from them. Genes can influence brain development, brain chemistry, and stress responses. These, in turn, affect learning, emotion, and decision-making. For example, a genetic tendency toward lower serotonin functioning may be linked with mood difficulties, while chronic stress hormones like cortisol may change how a person responds to future stress.

This topic also connects to animal research. Scientists sometimes study inherited behaviour patterns in animals because controlled breeding and observation can reveal patterns that are harder to study in humans. Animal research can support biological explanations, but psychologists must be cautious when applying animal findings to human behaviour, since humans have language, culture, and complex social life.

A good IB-style response should show balance. Instead of saying genes “cause” behaviour on their own, explain that genes contribute to risk, influence biological systems, and interact with environment. This kind of answer shows understanding of both terminology and scientific reasoning.

Conclusion

students, genes are powerful biological influences, but they do not work alone. They shape proteins, brain development, and chemical systems that can affect behaviour. Yet environment, learning, stress, and support also matter. The strongest explanation is usually an interaction between inherited risk and life experience. In IB Psychology HL, this topic helps you understand why biological explanations are important, but also why they must be used carefully and in combination with environmental explanations. 🧬

Study Notes

• Genes are sections of DNA that contain instructions for making proteins.
• Behaviour is usually influenced by many genes, so most traits are polygenic.
• Genes can affect behaviour indirectly through the brain, neurotransmitters, hormones, and development.
• A predisposition is a tendency, not a guarantee.
• Gene-environment interaction means genetic effects depend on the environment.
• Gene-environment correlation means genes can shape the environments people experience.
• Twin studies compare similarity in identical and fraternal twins to estimate genetic influence.
• Adoption studies help separate inherited effects from environmental effects.
• Molecular genetics looks for specific genes or gene patterns linked to behaviour.
• Correlation does not prove causation.
• Genetic explanations are useful, but they must be balanced with environmental explanations.
• In the biological approach, genes connect to brain, hormones, and other physical systems that influence behaviour.