Finger Length

Finger Length Photo Credit: Clipart.com

Although life experience plays a huge role in shaping who we are, the foundations of our personality begin in the womb. One recent study looks to finger length for signs of a man's pre-natal exposure to testosterone.


Measuring aggression with a finger. I'm Bob Hirshon and this is Science Update.

In women, the index and ring finger are roughly equal in length. But in most men, the ring finger is longer. That's a result of fetal exposure to testosterone.

Psychologists Alison Bailey and Pete Hurd, of the University of Alberta in Canada, studied these finger ratios in male college students. And they found that men with more dramatic differences tended to be more aggressive.


Here we have a fairy subtle personality trait—these are all people who made it into university—but within that population there's a substantial proportion of that personality trait that seems to be determined in the womb.

Dr. Hurd says finger ratios also correlate with other sex-linked traits, including spatial reasoning, running speed, and autism. Next, he plans to find out if they can predict how much time hockey players spend in the penalty box. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

After hearing this report, you're probably looking at your own fingers. Before you leap to any conclusions, it's important to keep in mind a few things. First of all, none of the men in this study was an axe murderer. They were all ordinary college students. The subjects judged their own level of aggression by responding to a questionnaire. (In other studies, this questionnaire has proven fairly reliable.)

Second, having a larger finger-length ratio doesn't cause aggression. It's only correlated to aggression. When two things are correlated, you can make predictions about one based on the other. For example, if you look out your window and see people carrying umbrellas, there's a good chance the ground is wet. But they aren't carrying the umbrellas because the ground is wet, or vice versa. Wet ground and umbrella-carrying correlate because they share a common cause: rain.

It's like that with finger length, too. All male fetuses are exposed to testosterone in the womb; that's part of what makes them turn into boys instead of girls. Generally speaking, the more testosterone a male fetus receives, the more dramatic his finger-length ratio will be, and the more aggressive he will be as an adult. So, in this case, the researchers are using finger length as a quick and easy way of measuring how much testosterone a man was exposed to before birth.

That said, there are many other factors that contribute to aggression, many of which are genetic and many more of which come from one's environment. Clearly, a boy raised in an abusive home might have a good chance of becoming aggressive regardless of how much testosterone he got as a fetus. Overall, Hurd says that finger-length ratio (and, therefore, fetal testosterone exposure) can predict only about 10 percent of the variation in aggression across individuals.

Still, it's interesting to see that there are aspects of our personality that are sealed before we're even born. And as Hurd mentions, there are other typically male traits—from skills to diseases—that are associated with finger-length ratios as well. If these correlations are well established, scientists may be able to better understand the role of testosterone in shaping human beings.

Now try and answer these questions:

  1. How do the fingers of men and women differ in length?
  2. What does it mean to say that finger length and aggression are correlated? What does it not mean?
  3. Why are finger length and aggression related?
  4. Can you think of other correlations in everyday life? (Think back to the umbrella and wet ground example.)


Going Further

For Educators

The Psychology Today article Autism: What's Sex Got to Do With It?, discusses the potential link between autism and fetal testosterone.

On Nova Online's The Odyssey of Life, students can view a time-lapsed video of a human embryo developing in a womb.

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