December 8, 2012
Prenatal testosterone exposure, as indicated by relative finger length, may be a marker of increased verbal aggression in adults, new research suggests.
In 2 studies, investigators measured the ratio of length of the second digit/index finger to length of the fourth digit/ring finger (2D:4D) of more than 600 young adult volunteers.
Those who had smaller 2D:4D ratios, which correlates with prenatal exposure to testosterone, reported more verbal aggression behaviors than did the participants with higher ratios. In addition, the male participants showed smaller 2D:4D ratios and higher levels of verbal aggression than their female counterparts.
“These findings are very promising,” lead author Allison Shaw, PhD, assistant professor in the Department of Communication at the University at Buffalo–State University of New York, told Medscape Medical News.
The investigators report that this is one of the first studies to use this method to examine prenatal testosterone exposure as a determinant of a communication trait.
Although verbal aggression may be beneficial in certain situations, such as when standing up for oneself if attacked, higher degrees of this behavior have been shown to be detrimental, they note.
“Understanding the causes of verbal aggression, both biological and social, will allow therapists to have a greater understanding of how to work with these individuals,” said Dr. Shaw.
“In terms of clinical practice, I think the take-home message is that there is a longer process that is involved with this. It’s not just a set of behaviors.”
The study is published in the October issue of the Journal of Communications.
Proxy for Sex Hormones
According to the researchers, the ratio of 2D:4D is an indicator of prenatal androgen exposure (PNAE).
“The endocrine literature indicates that the ratio of the length of 2D to 4D is smaller for men than for women and this difference is driven by PNAE,” they write.
“Most importantly, data indicate that 2D:4D is a proxy for sex hormones levels at the time of brain organization.”
Previous research has also shown a link between 2D:4D and mental rotation ability, courtship behaviors, dominance, athletics, memory, and physical aggression.
“I became very interested in understanding how prenatal hormones can affect adult behavior. And as a communications major, I was especially interested in looking at communication behaviors,” said Dr. Shaw.
She noted that a recent study suggested that 2D:4D could predict financial success over a lifetime, which then gave her the idea to apply this technique toward understanding communication behaviors “not just in a social context but also within a biological one as well.”
In the first study, 224 students from the University of Tennessee in Knoxville (52% women; mean age, 20.2 years) had each hand photocopied. From these images, measurements were taken of each finger from its tip to where it meets the palm of the hand.
Questionnaires that included Infante and Wigley’s verbal aggression measure were then administered to all participants.
The second study included 405 students from a large Midwestern university (49.6% women; mean age, 20.4 years). Investigators measured each of the participants’ fingers in person and from images of their hands.
These students filled out the same verbal aggression measure used in the first study as well as the self-reported Infante and Rancer’s Argumentativeness scale and the HEXACO Personality Inventory.
In the first study, the men’s 2D:4D ratio was significantly smaller than the women’s — but only on the right hand ( P = .005). The men also showed higher levels of verbal aggression than did the women ( P < .001).
In addition, there were statistically significant correlations between 2D:4D and verbal aggression for both hands in both the men and the women.
In the second study, the men had significantly smaller mean 2D:4D ratios than the women on both hands for both the in-person and the photocopied measures. These men were also statistically more verbally aggressive than the women, but they were less argumentative.
Finally, the higher the level of verbal aggression, the lower the 2D:4D ratio for both sexes for the live measure of the right hand and photocopies of both hands.
The ratio did not correlate with either argumentativeness or openness to other experiences.
“This second study showed that 2D:4D didn’t correlate with just any type of communication behavior. Instead, it was with a very specific behavior caused by prenatal testosterone exposure,” said Dr. Shaw.
“Future research would profit by attempting to explicate the mediating mechanisms that result in androgen exposure and differences in 2D:4D and psychological dispositions,” write the investigators.
Dr. Shaw noted that, even so, the difference between the second and fourth digits for everyone “is pretty small.”
“You can’t really look at your hand and know your ratio or know if you’re predisposed to be more verbally aggressive than someone else,” she said.
“Instead, this is a proxy. In human research, we don’t have the ability to measure things perfectly. So these indicators are very important.”
J Commun. 2012;62:778-793. Abstract – The Effect of Prenatal Sex Hormones on the Development of Verbal Aggression
Earlier reports about 2D:4D digit ratios and agressesion:
After various studies in the past had already suggested that 2D:4D digit ratio could be linked with the Big Five personality dimension Extraversion / Introversion, a new experimental study shows how the finger ratio combined with hand shape produces highly significant results.
The picture above displays a schematic presentation of the results: e.g. the ‘+’ and ‘-‘ signs show the proportional tendencies in the hands of Extraverts and Introverts. Details regarding the underlying results are presented in the article: