The New Brain

How your brain—and our understanding of it—are constantly changing.

How to Measure Propensity for Aggression with a 6-Inch Ruler

Measurements of a man's hand can predict aggression and dominance

Psychologists have many cleaver tests to ascertain personality traits of people.  Now, according to a study published in the journal Aggressive Behavior, the shrinks have a new tool in their medical diagnostic kit: a 6-inch ruler.

Leander van der Meij and colleagues at the Department of Psychology in The Netherlands report a finding that seems more rooted in mysticism of the palm reading parlor than in science from a laboratory, but there is good biological evidence to back up their conclusion: Two measurements of a man's hand can predict how aggressive and domineering he will be.

To test yourself, simply measure the length of your pointing finger and your ring finger (the second and fourth digit). Measure from the crease where these fingers join the palm to the finger tips. Now divide your pointing finger length by your ring finger length to calculate the 2D:4D ratio. The ratio ranges from about 0.9 to 1.03. The lower the ratio, the higher your aggressive dominance. 

Palm reading to measure personality? How can that be? 

Here's a clue. The study involved only men: 84 university students between the ages of 18 and 29, who were given psychological tests for aggressive dominance and then had their fingers measured.  That's because women have a higher 2D:4D ratio than men; that is, in men the ring finger tends to be significantly longer than the pointing finger, but in women these two fingers are closer to the same length. Lesbians, however, have a more "masculine" 2D:4D ratio, according to a 2010 study by Grimbos and colleagues. Another clue is that girls who share the womb with a twin brother are more prone to aggressive behavior than girls who have a twin sister, according to Cohen-Bendahan in a 2005 study. Even in the animal world aggressive dominance in monkeys is related to a more masculine 2D:4D ratio. 

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The biological underpinning of this finding relates to the amount of testosterone bathing the fetus in utero. In people, and in animal studies, the higher the testosterone levels in utero, the smaller the 2D:4D ratio of the fingers.  This is thought to be related to a master gene, called Hox, which controls development of several aspects of fetal development, including the hands, the sexually dimorphic regions of the brain, and the genitalia. (A previous study found a correlation between penis length and the 2D:4D ratio. That study used similar methods but a somewhat longer ruler.) Other personality traits also correlate with a more masculine 2D:4D ratio, including a higher propensity for sensation seeking, a shorter length of intimate relationships in women, greater courtship display in men, and worse spatial navigation in women. 

The study is food for thought on the relationship between genes and environment, personality and free will, gender and the complex mix of biology, society, and psychology. 

Now, perhaps we know why the ring goes on the fourth finger. Take a good look!  The studies make no mention of the palmist's life line, however.

R. Douglas Fields, Ph.D., is the Chief of the Nervous System Development and Plasticity Section at the National Institute of Child Health and Human Development and the author of The Other Brain. more...

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