Introduction to Biological Research Methods

In Biological Psychology, we are interested in how the physical body—especially the brain and our genetics—influences our behaviour, such as aggression. To study this, psychologists can't just watch people; they need specialized tools to look "under the hood." In this chapter, we will explore correlations (to see how things relate), brain-scanning (to see the brain in action), and twin/adoption studies (to see if behaviour is born or bred).

1. Correlational Research

A correlation is a way of measuring the relationship between two variables. In an experiment, we change an Independent Variable (IV) to see the effect on a Dependent Variable (DV). However, in a correlation, we simply measure two co-variables to see if they change together.

Types of Correlation

  • Positive Correlation: As one co-variable increases, the other also increases. Example: As testosterone levels go up, levels of verbal aggression also go up.
  • Negative Correlation: As one co-variable increases, the other decreases. Example: As the amount of a specific neurotransmitter increases, the number of aggressive incidents decreases.
  • Zero Correlation: There is no relationship between the variables at all.

Visualising Data: Scatter Diagrams

We plot correlational data on a scatter diagram. Each dot represents one participant's score for both co-variables. If the dots form a line pointing "up" to the right, it’s positive. If they point "down" to the right, it’s negative.

Hypotheses in Correlations

When writing a hypothesis for a correlation, we don't use the words "cause" or "difference." Instead, we talk about the relationship:

  • Null Hypothesis: States there will be no significant relationship between the two co-variables.
  • Alternate Hypothesis: States there will be a relationship. This can be directional (predicting it will be positive or negative) or non-directional (just saying there will be a relationship).

The "Cause and Effect" Trap

Important: A correlation does not prove that one thing causes another. It only shows they are related. There might be a third variable (an intervening variable) that causes both. For example, heat might be correlated with ice cream sales and aggression, but ice cream doesn't cause aggression!

2. Statistics: Spearman’s Rho

  • The result (the observed value) will always be between \(+1.0\) (perfect positive) and \(-1.0\) (perfect negative).
  • To see if your result is significant, you compare it to a critical value from a statistical table.
  • For Spearman’s rho, your observed value must be equal to or greater than the critical value for it to be significant.
  • We usually look for a significance level of \(p \leq .05\). This means there is a 5% or lower possibility that the results happened by chance.

3. Brain-Scanning Techniques

Biological psychologists use scans to investigate how brain structure and brain function relate to behaviours like aggression.

CAT Scans (Computerised Axial Tomography)

CAT scans use X-rays to create a 3D image of the brain's structure. They are great for spotting physical damage or tumours in specific areas like the pre-frontal cortex.

PET Scans (Positron Emission Tomography)

PET scans show brain function. The patient is injected with a slightly radioactive tracer (like glucose). The scan shows which parts of the brain are "using" that energy.
Link to Study: Raine et al. (1997) used PET scans to show that murderers had lower activity in their pre-frontal cortex compared to non-murderers.

fMRI Scans (Functional Magnetic Resonance Imaging)

fMRI scans measure brain activity by detecting changes in blood oxygenation. When a brain area is active, it consumes more oxygen. These scans are very detailed and show the brain "working" in real-time without using radiation.

Did you know? fMRI scans are often preferred over PET scans because they are non-invasive (no injections) and provide very high-resolution images of brain activity!

4. Twin and Adoption Studies

These methods help psychologists solve the Nature vs. Nurture debate. They want to know: is aggression something we inherit through our genes, or something we learn from our environment?

Twin Studies

Psychologists compare two types of twins:

  • MZ (Monozygotic): Identical twins who share 100% of their genes.
  • DZ (Dizygotic): Non-identical twins who share about 50% of their genes (just like regular siblings).

We look for the concordance rate (the percentage of twin pairs where both twins show the same behaviour). If MZ twins have a much higher concordance rate for aggression than DZ twins, it suggests aggression has a genetic (nature) basis.
Example: Brendgen et al. (2005) studied 6-year-old twins to see how much social aggression was due to genes vs. environment.

Adoption Studies

Adoption studies are powerful because they separate "nature" from "nurture." If an adopted child behaves more like their biological parents (whom they've never met), the cause is likely genetic. If they behave more like their adoptive parents, the cause is environmental.

Key Takeaway: Twin studies compare MZ and DZ twins to find genetic links. Adoption studies compare children to biological vs. adoptive parents to separate genes from upbringing.

Summary Checklist

Before moving on, make sure you can:

  • Define positive and negative correlations.
  • Explain why correlations cannot show cause and effect.
  • Identify the observed value and critical value in Spearman's rho.
  • Describe how CAT, PET, and fMRI scans work.
  • Explain the difference between MZ and DZ twins in research.

Note: For more details on specific research, see the "Prescribed studies: biological psychology" chapter, which covers Raine et al. (1997) and Brendgen et al. (2005) in depth.