Hormones, genes and twin studies of aggression

Introduction: Nature or Nurture?

Have you ever wondered why some people seem to have a "shorter fuse" than others? In Biological Psychology, we look at whether aggression is something we are born with (our genes) or something caused by the chemicals inside us (our hormones). This chapter explores how these biological factors influence our behavior and how psychologists use twin studies to prove that aggression might be passed down through families.

1. The Role of Hormones in Aggression

Hormones are chemical messengers sent through the bloodstream. While many hormones exist, two are particularly important when studying aggression: Testosterone and Cortisol.

Testosterone: The "Drive" Hormone

Testosterone is an androgen (male sex hormone), though it is present in both males and females. It is linked to dominance and competitiveness.
- High levels of testosterone are often associated with increased physical aggression.
- Research suggests that testosterone influences the areas of the brain that control emotional responses (like the amygdala), making a person more likely to react aggressively to a threat.

Cortisol: The "Stress" Hormone

Cortisol is usually released in response to stress. Interestingly, it seems to have an inverse relationship with aggression:
- Low levels of cortisol are often found in people who are highly aggressive or have conduct disorders.
- The "Brake" Analogy: Think of testosterone as the "accelerator" for aggression and cortisol as the "brake." When cortisol is high, it tends to inhibit (stop) aggressive behavior. When cortisol is low, the "brake" isn't working, making the person more likely to act on their aggressive impulses.

Quick Tip: Remember that hormones don't cause aggression on their own, but they make it more likely that someone will react aggressively in a stressful situation.

2. The Role of Genes in Aggression

If aggression is biological, it should be passed down from parents to children through DNA. Psychologists have identified specific genes that might be responsible.

The MAOA Gene (The "Warrior Gene")

The MAOA gene produces an enzyme (Monoamine Oxidase A) that breaks down neurotransmitters like serotonin and dopamine in the brain.
- If a person has a low-activity version of this gene, the enzyme doesn't work efficiently.
- This causes a "buildup" of neurotransmitters, which can lead to impulsive and violent behavior.
- Analogy: Imagine the MAOA gene is a recycling truck. If the truck breaks down, "trash" (excess neurotransmitters) piles up in the street, causing a mess (aggression).

3. Twin Studies: Testing Genetic Relatedness

How do we know if aggression is truly genetic or just learned from parents? We look at twins! Twin studies are a "natural experiment" because they compare two types of twins:

1. Monozygotic (MZ) Twins: These are identical twins. They share \(100\%\) of their genes.
2. Dizygotic (DZ) Twins: These are non-identical (fraternal) twins. They share \(50\%\) of their genes, just like regular siblings.

Understanding Concordance Rates

Psychologists look for concordance rates. This is the percentage of twin pairs where both twins show the same trait (like aggression).
- If MZ twins have a much higher concordance rate for aggression than DZ twins, it suggests that genes are a major cause, because MZ twins are genetically identical.
- If the rates are similar, it suggests the environment (the way they were raised) is more important.

Key Takeaway: Twin studies help us calculate the "heritability" of a trait—how much of the difference between people is due to their DNA.

4. Contemporary Research: Brendgen et al. (2005)

In your exam, you need to know about Brendgen et al. (2005). This study investigated 6-year-old twins to see if aggression was due to genes or the environment. They looked at two types of aggression:

- Physical Aggression: Hitting, biting, or kicking.
- Social Aggression: Spreading rumors, excluding others, or "mean girl" behavior.

What did they find?

- Physical aggression had a high concordance rate in MZ twins, suggesting it is mostly genetic.
- Social aggression was more similar in both MZ and DZ twins, suggesting it is mostly environmental (learned from friends or parents).
- They also found that children with high physical aggression often "moved into" social aggression as they got older, perhaps because they learned that social exclusion is more socially acceptable than hitting.

5. Evaluating the Biological Explanation

Don't worry if this seems a bit one-sided; in Psychology, we always look at the strengths and weaknesses!

Strengths:
- Scientific Credibility: Using blood tests for hormones and DNA for genes is objective and reliable.
- Real-world application: If we know certain genes or hormones lead to aggression, we might be able to develop treatments or interventions.

Weaknesses:
- Reductionism: It simplifies a complex human behavior down to just "chemicals" and "DNA," ignoring the role of upbringing, poverty, or social learning.
- Correlational: Just because a person has high testosterone doesn't mean it caused the aggression. Maybe winning a fight caused the testosterone to rise!

Quick Review Box

Hormones: High Testosterone \(+\) Low Cortisol \(\implies\) Higher Aggression.
Genes: The MAOA gene (low activity) is linked to impulsivity.
Twin Studies: MZ twins (\(100\%\) shared genes) vs. DZ twins (\(50\%\) shared genes). Higher similarity in MZ twins suggests a genetic cause.
Brendgen (2005): Physical aggression is mostly nature (genes); social aggression is mostly nurture (environment).

Note: For more information on how we see these biological changes in the brain, refer to the "Brain-scanning techniques" and "Brain structure" chapters.