What can be inherited?

Genetic inheritance is the transmission of genetic information from biological parents to their offspring. A gene is a segment of DNA that contributes to a functional product, such as a protein. Through these products, genes can influence how biological systems involved in behaviour develop and function.

Inheriting a biological predisposition is different from inheriting a finished behaviour. A person does not inherit a particular argument or a decision to avoid a crowded room. They may, for example, inherit characteristics that affect how readily they respond to threat. Experience then shapes when and how that response appears.

Many genes influence most complex human behaviours, with each usually contributing a small effect; one gene rarely acts alone. Those genetic influences operate within an environment. Nutrition and relationships can affect the eventual outcome, as can learning and stressful experiences.

Establishing the extent of genetic influence

Family resemblance gives us a starting point, not proof of inheritance. Relatives share aspects of their environment as well as genes.

Comparing twins helps separate these possibilities. Identical twins share nearly all their inherited genetic variation; non-identical twins share about half on average. If identical twins show greater behavioural similarity, that is consistent with a genetic contribution. Adoption comparisons offer another approach: similarity to biological relatives may support genetic influence, while similarity to adoptive relatives may support environmental influence.

But these comparisons depend on assumptions. Identical twins may be treated more similarly than non-identical twins. Adoption does not remove every environmental similarity with biological relatives, either. Choosing a particular sample therefore does not, by itself, cleanly separate genetic and environmental explanations.

Heritability is a population statistic that estimates the proportion of variation in a characteristic associated with genetic differences under particular environmental conditions. It doesn't tell us what proportion of one person's behaviour is genetic. High heritability does not mean that a characteristic cannot change.

We can defensibly say that behaviour can be partly inherited. How much depends on the behaviour and the population, as well as the environment being considered. Genetic influence is not genetic inevitability.

How a chemical can influence behaviour

Cells release a chemical messenger to communicate information to other cells through suitable receptors. A receptor is a protein that responds to a particular chemical signal and helps change a cell's activity. The same messenger won't affect every cell equally. A target cell must have an appropriate receptor.

Two relevant categories are neurotransmitters and hormones. Neurons release neurotransmitters to transmit signals across synapses. Endocrine cells release hormones, which travel through the bloodstream to target cells. Neurotransmitters typically signal rapidly and locally; hormones can coordinate responses across several organs, often with longer-lasting effects. Some substances take on different signalling roles depending on where and how they are released.

Dopamine and motivated behaviour

Dopamine is a neurotransmitter involved in movement, motivation and learning about rewards. In some neural pathways, dopamine signalling helps a person learn which cues and actions predict rewarding outcomes. These learned expectations can shape future choices and how much effort someone puts into obtaining an outcome.

Consider someone who repeatedly receives encouraging feedback after practising a skill. They may become more willing to practise again. Dopamine-related reward learning can contribute to this process, though how the person interprets the feedback and their previous experiences matter too. Calling dopamine a 'pleasure chemical' misses much of its role. Wanting, learning and enjoying are not identical processes.

Cortisol and responses to stress

The adrenal cortex releases Cortisol, a hormone that helps regulate the body's response to demands, including stress. It helps make energy available and influences processes involved in attention and memory.

An acute response may help someone cope with a demanding situation. When exposure is prolonged or poorly regulated, it can interfere with aspects of memory and contribute to changes in behaviour. Timing, duration and the situation affect the outcome; more cortisol does not produce one fixed behavioural outcome.

To explain either messenger's influence on behaviour, follow the chain: the chemical is released, acts at suitable receptors and changes biological activity, which then influences behaviour. A chemical concentration alone cannot fully explain a person's actions.

From an electrical signal to a chemical signal

Through neurotransmission, a neuron communicates with another cell across a synapse using a neurotransmitter. A neuron is a specialised cell that receives and transmits information through electrical and chemical signals. The junction where it communicates with another cell is a synapse.

At a typical chemical synapse, an electrical signal travels along the sending neuron. It then crosses the gap as a chemical signal, changing the activity of the receiving cell. The diagram shows the stages in their correct order.

An action potential is a brief electrical change in a neuron's membrane that propagates along its axon. On reaching the axon terminal, it opens channels that let calcium ions enter. Their entry triggers vesicles—small membrane-bound sacs—to fuse with the terminal membrane, releasing neurotransmitter into the synaptic gap.

Once released, the neurotransmitter diffuses across the gap and binds to receptors on the receiving cell. Activating these receptors changes the cell's activity. The sending side is called presynaptic; the receiving side is called postsynaptic.

Excitation, inhibition and ending the signal

An excitatory effect makes a receiving neuron more likely to generate an action potential. A change that makes it less likely to do so is an inhibitory effect. Each neuron integrates many incoming signals. One neurotransmitter molecule arriving at one receptor doesn't automatically make it fire.

The receptor and the receiving cell determine the effect, not just the neurotransmitter's name. Calling a messenger universally excitatory or inhibitory can therefore be misleading.

Signalling must end, too. Through reuptake, a transport process that removes released neurotransmitter from the synaptic gap, neurotransmitter may be taken back into the sending neuron. Depending on the messenger, enzymes may also break it down, or it may diffuse away.

Connecting the synapse to behaviour

Changing neurotransmitter release, receptor activity or signal removal can alter how neural networks communicate. These changes may influence reward learning, attention or movement, for example, and affect behaviour through those processes.

A substance that slows a messenger's reuptake can keep it available at a synapse for longer. It doesn't simply increase a behaviour by the same amount. The effects depend on the messenger, the receptors and the networks involved. The synapse supplies a mechanism; the network and the situation help explain the behavioural outcome.

Vulnerability and environmental demands work together

The diathesis-stress model explains how a pre-existing vulnerability interacts with environmental stress to influence the likelihood of a psychological difficulty. A diathesis is a predisposition that makes someone more susceptible to a particular difficulty. Here, the predisposition is genetic. Vulnerabilities in the wider model, though, need not be exclusively genetic.

When people perceive demands as taxing or exceeding their coping resources, their psychological and physiological response is called Stress. A stressor is an event or condition that can provoke this response. The distinction is straightforward: a stressor is the demand; stress is the response to it.

The model connects inherited vulnerability with environmental demands to explain the likelihood of an outcome. Protective influences can change that relationship—they don't simply sit outside it.

What interaction means

An interaction occurs when the influence of one factor depends on the other. Someone with greater inherited vulnerability may be more strongly affected by a stressful experience than someone with lower vulnerability. In a supportive environment, by contrast, an inherited vulnerability may remain relatively unexpressed.

Imagine two people facing a prolonged period of uncertainty at work. One has greater inherited susceptibility to anxiety-related difficulties. According to the model, the same demand may increase this person's risk more strongly. That doesn't mean this person must develop a difficulty, or that the other person cannot.

Protective factors are characteristics or conditions that reduce the likelihood or impact of an adverse outcome. Even when vulnerability is present, reliable social support and effective coping may reduce the effect of stress. Access to help may do so too.

What the model adds—and what it does not establish

Rather than treating genes and environment as competing explanations, the model brings them together. It explains why a predisposition need not produce an outcome in every person, and why people can respond differently to the same environment.

Still, researchers must measure vulnerability and stress clearly, rather than infer them after a difficulty appears. The model's broad logic does not identify the genes or stressors that matter in a particular case, nor the mechanisms involved. Finding both genetic and environmental associations is not, by itself, proof that they interact.

Explaining behaviour at a smaller scale

Biological reductionism explains complex behaviour primarily through lower-level biological processes, such as genes, chemical messengers or neural activity. The explanation shifts to a smaller scale: researchers examine an experience involving a whole person through its biological components.

Consider persistent avoidance. An explanation based mainly on inherited susceptibility and chemical signalling is biologically reductionist if it pays little attention to learned expectations, the meaning of the situation or social conditions. Studying biology isn't automatically reductionist. The issue is whether the explanation treats biological processes as sufficient to account for the behaviour.

Strengths of the approach

A broad question about behaviour can become a precise, testable one through biological reductionism. Researchers might investigate a particular signalling process or compare a measurable biological characteristic across groups. Clear operational measures make findings easier to scrutinise and replicate, though measurement quality still matters.

The approach can also reveal mechanisms. Showing how a change in signalling alters a behavioural process explains more than simply reporting that two characteristics occur together. When ethical and practical, manipulating a biological process can strengthen causal reasoning.

Biological explanations may support targeted interventions too. If a signalling pathway contributes to a difficulty, altering that pathway may help. A useful intervention is a genuine strength. Successful treatment, however, does not prove that the difficulty had only a biological cause.

Limitations of the approach

Behaviour depends on biological systems interacting with individual experience and social circumstances. A focus on one gene or messenger can leave out the processes connecting biology to the behaviour being explained. Even when a biological measure is associated with a difficulty, it could be a cause, a consequence or a correlate of another factor.

Meaning can get lost in a reductionist explanation. The biological activity accompanying fear does not explain why a particular situation frightens that person. Situations with very different personal significance may produce the same physiological response.

An overly narrow account may encourage the mistaken belief that behaviour is fixed because biology contributes to it. Environmental conditions can affect both inherited susceptibility and chemical signalling; biological does not mean unchangeable.

Judging the approach therefore calls for a conditional answer. Biological reductionism is valuable for isolating mechanisms and developing interventions. It is less adequate as a complete explanation when learning, interpretation and social conditions are essential to the outcome. The diathesis-stress model illustrates how an interaction-based account can retain biological detail within a broader explanation.