Two directions of influence
Behavior depends on the brain, and the brain is shaped by behavior. Both statements are well supported, and keeping both in mind prevents a lot of confusion. A person’s actions, feelings and choices rely on physical activity in the nervous system. At the same time, experiences, habits and surroundings leave traces in the brain over time. So when a study reports a difference in the brain between two groups of people, there are many possible explanations for how it got there.
What the brain contributes
Several levels of brain organization have been linked to behavior.
Structure. The size, shape and connections of brain areas differ between people. Damage to particular regions, from stroke or injury, can cause specific changes in behavior. A famous nineteenth-century case, a railway worker named Phineas Gage who survived an iron rod passing through his frontal region, is often cited as showing that injury to the front of the brain can alter personality, though the accounts of his actual changes were exaggerated over time and are uncertain in their details.
Chemistry. Neurotransmitters such as dopamine and serotonin carry signals between neurons, and medications often work by changing their levels or effects. But popular labels like the pleasure chemical or the happiness chemical are oversimplifications. Dopamine, for example, is involved in movement, motivation and learning, among other roles, which is why its role in the brain reward system is more complicated than the slogans suggest.
Activity and networks. Imaging studies show which areas tend to become more active during tasks, and how different regions fluctuate together. These patterns are associated with attention, emotion, memory and decision making, and the study of emotion circuits illustrates how several areas cooperate.
Hormones and body state. Hormones, sleep, illness and fatigue influence the brain too. Behavior is never purely a brain affair: the brain operates in a body and an environment.
Why correlation is not enough
A frequent type of finding is that some brain measure is related to some trait. For example, a researcher might report that people with a larger volume in one region score higher on a test, or that people with a certain condition show unusual activity in a network. Such results are correlations, and they leave open several possibilities, as the general explanation of correlation and causation makes clear.
- The brain feature may contribute to the behavior.
- The behavior may change the brain feature, as when long practice at a skill alters related areas.
- A third factor, such as age, sleep, medication, stress, or education, may influence both.
- The relationship may be partly or entirely a statistical accident, especially in small samples.
Telling these apart requires different kinds of evidence: studies that follow people over time, experiments that change something and observe the result, studies of people with specific injuries, and repeated tests in new groups. Even then, conclusions are usually probabilistic.
Group averages and individuals
Another caution is that findings usually describe averages. A difference in average brain activity between two groups can be real while the two groups overlap heavily, so the finding cannot reliably classify any particular person. This is part of why brain scans are not used to read someone’s character or predict future behavior, and why claims that a scan can reveal who is honest, creative or likely to commit a crime deserve skepticism. The topic of individual differences takes up how people vary.
Simple stories and the real picture
The temptation to link one behavior to one spot or one chemical is strong. Popular articles often say that a region is the center for fear, love or decision making. The truth is that many functions are distributed over networks, and a given region contributes to many different tasks. The overview of brain regions explains how to treat such labels as rough guides.
Similarly, the idea of the left brain being logical and the right brain creative does not hold up well. The two hemispheres do show some specialization, language being the best known example in most people, but healthy brains use both hemispheres together for almost everything.
Environment and development
Genes influence brain development, but so do nutrition, stress, social contact and learning, and the effect of any one factor is typically modest and context dependent. Early life experiences matter, but brains continue to change in adulthood, as in the discussion of brain plasticity. Words like hardwired can mislead if taken to mean fixed.
Using this knowledge sensibly
Understanding the relationship between brain and behavior can encourage compassion: conditions such as depression, addiction and attention difficulties involve real biology, and are not simple failures of character. It does not follow that biology alone explains everything, or that biology means a condition cannot change. Treatments and supports can operate at many levels, from medication to therapy to changes in routine.
When you meet a headline like scientists find the brain area behind some trait, a few questions help. How many people were studied? Was it a correlation or an experiment? Did the finding hold up in other groups? And does the claim go further than the measurement? Applying these questions will not make you an expert, but it can make you a more careful reader of research on the mind.