What you'll learn

• How Darwin’s idea of natural selection can be used to explain behaviour.
• Why evolutionary explanations focus on survival and reproductive success.
• How aggression may have been adaptive in some ancestral contexts.
• How to evaluate evolutionary explanations using evidence, methodology, ethics, and real-world application.

The big idea: why evolution matters in psychology

Evolutionary explanations ask: why might a behaviour have helped humans survive or reproduce in the past?

Charles Darwin (1859) argued that species change over time because some inherited characteristics make survival and reproduction more likely. Psychology applies this logic to behaviour: if a behaviour helped our ancestors solve important problems, the biological tendency for that behaviour may have become more common over generations.

This does not mean every behaviour is directly caused by genes, or that humans are “programmed” to behave in one fixed way. It means humans may have inherited predispositions: tendencies that make some behaviours more likely in particular situations.

The logic of natural selection

To understand natural selection, build it in small steps.

A trait is a characteristic of an organism. Traits can be physical, such as height, or behavioural, such as a tendency to respond quickly to threat. Genes are sections of DNA that influence characteristics, although most human behaviours are affected by many genes and by the environment.

A selection pressure is a challenge in the environment that affects survival or reproduction. Examples include predators, food shortages, competition for mates, or threats from rival groups.

Natural selection is easiest to see as a chain: people vary, some variations fit the environment better, those individuals are more likely to survive and reproduce, and their traits are more likely to be passed on.

Adaptations and reproductive fitness

An adaptation is an inherited characteristic that has been shaped by selection because it helped ancestors survive or reproduce. In psychology, an adaptation might be a mental mechanism, such as fear of danger, preference for high-energy food, attachment to caregivers, or rapid detection of social threat.

Reproductive fitness means success in passing genes to future generations. This can happen directly, by having offspring, or indirectly, by helping close relatives survive because relatives share some genes. This indirect route is called inclusive fitness.

Ultimate and proximate explanations

Evolutionary explanations are often called ultimate explanations because they ask what a behaviour may have been for in terms of evolutionary function.

A proximate explanation focuses on the immediate cause of behaviour, such as brain activity, hormones, neurotransmitters, learning, or social cues.

Both levels can be true at the same time.

Evolutionary explanations of aggression

Aggression is behaviour intended to cause physical or psychological harm. In evolutionary psychology, aggression is usually explained as a conditional strategy: it may be more likely when the potential benefits outweigh the costs.

Aggression is costly. It can lead to injury, retaliation, punishment, or exclusion from the group. So an evolutionary explanation does not predict constant aggression. It predicts aggression in contexts where it may historically have improved survival or reproductive success.

Resource defence

Aggression may have helped ancestors defend valuable resources such as food, territory, shelter, or tools. Someone able to deter theft or invasion may have improved their own survival and the survival of dependants.

Protection of self, offspring, and kin

Aggression may also be explained through protection. If a threat endangered offspring or close relatives, defensive aggression could increase inclusive fitness because protecting relatives also protects shared genes.

Status and rivalry

In some social groups, status affects access to resources, allies, and mates. Aggression may have evolved partly as a way to deter rivals or avoid being exploited.

Wilson and Daly (1985) described a “young male syndrome”, arguing that young men may be more likely to take risks and use violence in status competition, especially when they have less to lose. Daly and Wilson (1988) also used homicide evidence to argue that male-male violence often relates to rivalry, status, and sexual jealousy.

Sexual selection and mate retention

Sexual selection is a form of selection where traits evolve because they increase mating success. Intrasexual competition means competition within one sex, often over access to mates. In evolutionary explanations of aggression, this is often used to explain male rivalry or mate-guarding behaviour.

Buss et al. (1992) linked sexual jealousy to mate retention, suggesting that jealousy may have evolved because ancestral humans faced reproductive threats such as infidelity or loss of a partner. However, this is socially sensitive and must be treated carefully.

Evidence you can use

Darwin (1859) provides the foundation: inherited traits that increase survival and reproduction become more common. Lorenz (1966) later argued that aggression in animals can have adaptive functions such as spacing individuals out and protecting territory, though applying animal behaviour directly to humans is a weakness.

Daly and Wilson (1988) provide useful evidence because homicide patterns often involve young males, rivalry, and jealousy. This supports the idea that some forms of aggression may be linked to evolved pressures around status and reproduction.

Buss et al. (1992) can be used for sexual jealousy and mate retention, although findings about sex differences in jealousy are debated and may be influenced by culture, social norms, and measurement methods.

Ethically, research on aggression, jealousy, and sex differences is socially sensitive. Under the BPS Code of Ethics and Conduct (2009), researchers must consider consent, confidentiality, protection from harm, right to withdraw, and debriefing. Archival research using homicide records must be anonymised and reported respectfully to avoid harm to victims’ families or reinforcing stereotypes.

Evaluating evolutionary explanations

Strengths

Evolutionary explanations are useful because they explain why some behaviours appear across cultures and history. If similar patterns of rivalry, jealousy, or defensive aggression appear in many societies, this may suggest a shared evolved basis.

They also fit well with biological psychology. For example, aggression can be studied through hormones, brain regions, genetics, and neurotransmitters as proximate mechanisms, while evolution explains why those mechanisms may exist.

A further strength is real-world application. Understanding status threat, jealousy, and resource competition can help design violence-reduction strategies, such as de-escalation training, conflict mediation, and interventions that reduce humiliation or rivalry.

Weaknesses

A major weakness is that evolutionary explanations can be difficult to test directly. We cannot observe ancestral environments, so explanations may become “just-so stories”: plausible accounts that are hard to prove or falsify.

Another weakness is reductionism. Aggression is not caused by evolution alone. Learning, culture, family environment, social norms, alcohol, poverty, peer influence, and self-control all affect aggressive behaviour.

There is also contradictory evidence. Levels and forms of aggression vary greatly between societies and across historical periods. This suggests that culture and environment strongly shape whether any evolved predispositions are expressed.

Finally, evolutionary explanations can be socially sensitive. If presented badly, they may seem to excuse violence or reinforce gender stereotypes. In an exam, make it clear that explaining possible origins of aggression does not justify harmful behaviour.

Bringing AO1, AO2, and AO3 together

For AO1, describe the evolutionary logic: inherited variation, selection pressures, reproductive success, and traits becoming more common over generations.

For AO2, apply this to the scenario. If the question gives you aggression after jealousy, status threat, or resource competition, link the behaviour to an adaptive problem.

For AO3, evaluate with evidence and limitations. Use named research where possible, but avoid sounding as if evolution is the only explanation.