Asymmetry & Aggression

Asymmetry & Aggression

Much of the work of scientists is making connections in unexpected places. For example, a 2004 study found a link between aggression and body symmetry that may go back to the womb.


A strange connection between body and behavior. I'm Bob Hirshon and this is Science Update.

Scientists at Ohio State University recently published an unusual study. The researchers subjected college students to frustrating and belligerent phone calls. And they found that people with less symmetrical body features–for example, an arm, eye, or ear that's bigger than the other–tended to react more aggressively than their more symmetrical counterparts.

The study was led by Zeynep Benderlioglu. She says previous research has linked both asymmetry and aggression to risk factors in a mother's pregnancy.


For example, pregnant mothers who consume alcohol or tobacco, had more asymmetrical offspring compared to normal populations, who did not consume alcohol or tobacco during pregnancy. And mostly smoking and also alcohol use result in conduct disorders and aggression.

Building on that, her study is the first to experimentally link asymmetry and aggression to each other. The study doesn't suggest that asymmetry causes aggression, and can't prove that maternal risk factors are to blame. But it's one more clue that these seemingly unrelated traits may indeed share a common cause. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

This study nicely illustrates the difference between correlation and causation. Causation is a direct relationship between two things, where A causes B. For example, we know that smoking causes cancer, because of all the research that's gone into studying the way it happens. Causal relationships are usually (but not always) one-way: smoking causes cancer, but cancer does not cause smoking.

A correlation is simply a measurable and predictable relationship between two different things. For example, people who buy ashtrays probably have higher rates of lung cancer than people who don't. But that doesn't mean that buying ashtrays causes cancer, or the other way around. It simply means that if you know somebody buys ashtrays, you can predict that they will be more likely to get cancer than someone who doesn't. The reason, of course, is that smoking causes both cancer and ashtray-shopping.

So when two things are correlated, they may or may not also have a causal relationship. Correlations are a lot easier to prove, because all you have to do is measure the two things and study their relationship. They can provide clues that two seemingly separate phenomena may be connected, and point the way toward further study.

In this study, the researchers already assume that body asymmetry and aggression do not have a causal relationship. In other words, having asymmetric features doesn't make you short-tempered, and getting angry doesn't make your face asymmetrical. But previous studies had correlated a separate factor–stress from alcohol or tobacco during a mother's pregnancy–with both asymmetry and aggression.

Here, the researchers strengthen that relationship by connecting aggression to asymmetry. The underlying assumption of the study is that smoking and alcohol during pregnancy cause both aggression and asymmetry. This makes sense, because the pregnancy risk factors come first. But it's still possible that some other, unknown factor contributes to all three of these trends.

Now try and answer these questions:

  1. What is the difference between correlation and causation?
  2. What exactly is measured in this study?
  3. Correlation and causation are often confused in everyday life, in areas from science to politics to basic human interactions. Can you give an example?
  4. This study does not suggest that having asymmetrical features causes aggression. Would it be possible to test that idea directly? How?
  5. What are the dangers in publishing a study like this? How might it be misinterpreted? Do its potential benefits outweigh its drawbacks? Why?

For Educators

Correlation or Causation? is a big list of scientific studies that would serve as excellent topics for further discussion.

Beauty, by Australia Broacasting Company's Dr. Karl, and Looking Good, an article in the undergraduate-written Journal of Young Investigators, delve into the relationship between body symmetry and attractiveness.

Symmeter is a web-based system that provides a simple way to measure the symmetry of any person, place, or thing that can be rendered through a digital image.

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