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:
November 5, 2010
Researchers from England & Canada presented earlier this week brand new evidence that the neanderthals’ life was much more dominated by competition & promiscuity than our lifes today! Maybe more surprizing is the method that the researchers used to acquire their new findings: finger length ratio measurements!
The study, published in the Proceedings of the Royal Society B, draws upon a famous and controversial indicator of social behavior: the comparative length of the index finger and the ring finger, also known as the 2D:4D finger ratio. If the ring finger is longer than the index finger, that’s supposed to be correlated with higher prenatal exposure to androgens — resulting in a higher proclivity for aggressiveness and promiscuity.
A few numbers from the results:
• Modern humans averaged a 0.957 index-to-ring finger ratio, and were considered to be on the line between a “pair-bonded,” or monogamous, species and a middle-of-the-road species.
• Chimps, gorillas and orangutans had index-to-ring ratios in the 0.90 to 0.92 range, and were classified as “non-pair-bonded,” or promiscuous.
• An early modern human from Israel’s Qafzeh Cave, thought to be about 95,000 years old, had an index-to-ring ratio of 0.935. Based on that statistic, the researchers surmised this individual would be more promiscuous than modern humans.
• The finger bones from five Neanderthals yielded a 0.928 ratio, associated with even greater competitiveness and promiscuity. Ardipithecus’ bones took it up another notch, to 0.899. Two even older primate ancestors, Hispanopithecus and Pierolapithecus, had ratios of 0.848 and 0.908, which means they would have been tough to live with as well.
• On the other end of the spectrum, the monogamous gibbons had a 1.009 ratio … and the australopith sample came in with a ratio higher than that of modern-day humans (0.979). The implication, then, is that australopiths were monogamous.
Scientists, in collaboration with researchers at the universities of Southampton and Calgary, used finger ratios from fossilised skeletal remains of early apes and extinct hominins, as indicators of the levels of exposure species had to prenatal androgens – a group of hormones that is important in the development of masculine characteristics such as aggression and promiscuity.
It is thought that androgens, such as testosterone, affect finger length during development in the womb. High levels of the hormones increase the length of the fourth finger in comparison to the second finger, resulting in a low index to ring finger ratio (2D:4D digit ratio). Researchers analysed the fossil finger bone ratios of Neanderthals and early apes, as well as hominins, Ardipithecus ramidus and Australopithecus afarensis, to further understanding of their social behaviour.
The team found that the fossil finger ratios of Neanderthals, and early members of the human species, were lower than most living humans, which suggests that they had been exposed to high levels of prenatal androgens. This indicates that early humans were likely to be more competitive and promiscuous than people today.
The results also suggest that early hominin, Australopithecus – dating from approximately three to four million years ago – was likely to be monogamous, whereas the earlier Ardipithecus appears to have been highly promiscuous and more similar to living great apes. The research suggests that more fossils are needed to fully understand the social behaviour of these two groups.
Dr Susanne Shultz, from the Institute of Cognitive and Evolutionary Anthropology at the University of Oxford describes:
“Social behaviours are notoriously difficult to identify in the fossil record. Developing novel approaches, such as finger ratios, can help inform the current debate surrounding the social systems of the earliest human ancestors.”
And Dr Emma Nelson, an archaeologist from the University of Liverpool, argues that comparing the finger-length ratios of extinct and present-day species is a valid technique for making an indirect assessment of our long-gone ancestors’ social behavior. She said:
“It is believed that prenatal androgens (male sex hormones) affect the genes responsible for the development of the fingers, toes and the reproductive system. We have recently shown that promiscuous primate species have low index to ring finger ratios, while monogamous species have high ratios.”
“We used this information to estimate the social behaviour of extinct apes and hominins. Although the fossil record is limited for this period, and more fossils are needed to confirm our findings, this method could prove to be an exciting new way of understanding how our social behaviour has evolved.
Research at the universities of Liverpool and Oxford into the finger length of various primate species has revealed that cooperative, competive & sexual behavior is linked to exposure to hormone levels in the womb!
The British scientists used finger length ratio measurements as an indicator of the levels of exposure to the hormone and compared this data with social behaviour in primate groups.
Primates such as baboons and rhesus macaques, have a low ‘2D:4D digit ratio’ (= a longer fourth finger [ring finger] compared to the second finger [pointer finger]), and these species tend to be highly competitive and promiscuous.
While gibbons and many New World monkey species have higher ‘2D:4D digit ratio’ (but still lower than the average human digit ratio), and these primate species were monogamous and less competitive than Old World monkeys.
The results also show that Great Apes, such as orangutans and chimpanzees, expressed a different finger ratio. The analysis suggests that early androgen exposure is lower in this groups compared to Old World monkeys. Lower androgen levels could help explain why Great Apes show high levels of male cooperation and tolerance.
Primate researcher Emma Nelson explains:
“It is thought that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system. High androgen levels from a foetus or mother during pregnancy, may alter gene function and lead to subtle changes in relative digit length and the functioning of the reproductive system. Finger ratios do not change very much after birth and appear to tell us something about how very early androgens affect adult behaviour, particularly behaviour linked to mating and reproduction.”
ILLUSTRATION: A comparison of the human hand with primate hands reveals that only the human hand is featured with a long opposable thumb!
SUGGESTION FOR FURTHER READING:
March 13, 2009
John Manning is a professor at the University of Swansea and he wrote the book “The Finger Book”. Professor John Manning explains in his book how the ratio of the index finger and ring finger (= the ‘2D:4D digit ratio’ finger length) is related to the ‘Big five’ of life.
|FINGERS & EVOLUTION
The human thumb is known as a ‘marker’ for the evolution of human kind; however our relative long index finger – compared to the ring finger – is a likewise example. In the chimpanzee and gorilla, this hand feature is different: they always have longer ring fingers and a short index finger.
THE RING FINGER & SPORTS
Exceptional performances in sports are being linked to the amount of male hormones that people absorb before birth while they stay in the womb. The ‘digit ratio’ has frequently been used to predict performances in various sports leagues.
THE INDEX FINGER & SOCIAL BEHAVIOR
Interestingly, especially among children a short index finger may indicate a lack of empathic ability.
There is growing statistical evidence that our finger ratios are a reliable predictor of our receptiveness to diseases. However, in real life the findings have not yet shown to have a significant impact.
YOUR FINGERS & YOUR SEX-LIFE
Men with long ring fingers, consider themselves as attractive. Studies show that women in the general rule agree and confirm the judgements of these men. Surprisingly, a likewise result has been found in women.
READ FURTHER ABOUT MANNING’S FINDINGS:
September 29, 2008
Digit Ratio: A Pointer to Fertility, Behavior and Health – author: John T. Manning
Publisher: Rutgers University Press 2002
Review by Michael Mills, Psychology Department, Los Angeles.
|Michael Mills writes about John Manning’s book Digit Ratio:
“Take a look at your right-hand. Which of your fingers is longer: your ring finger, or your index finger? Surprisingly, a passing stranger who noticed a difference in length between these two fingers (and who had handy a copy of John Manning’s book Digit Ratio: A Pointer to Fertility, Behavior and Health) might infer some very personal characteristics about you. With no more data than that gleaned from a passing glance at your hands, a stranger might infer whether you are likely to have homosexual inclinations, are highly fertile, may eventually suffer from a heart attack or breast cancer, have musical aptitude or sporting prowess, and a surprisingly long list of other characteristics.
…Females typically have index and ring fingers of about the same length. The ratio of index finger length to ring finger length is called the “2D:4D digit ratio,” or more simply, the “digit ratio.” Manning reports that, for males, the index finger is generally about 96 percent of the length of the ring finger, which gives an average digit ratio for males of .96.
…Manning devotes separate chapters to explore the relationship between digit ratio and a variety of characteristics, including assertiveness and attractiveness (chapter 3), reproductive success (chapter 4), hand preference, verbal fluency, autism, and depression (chapter 5), health and disease (chapter 6), homosexuality (chapter 7), musical aptitude (chapter 8 ) and sports aptitude (chapter 9). A brief summary Manning’s findings (some of which he notes are quite preliminary) is presented in the table below.”
Low 2D:4D ratio
High 2D:4D ratio
* More fertile
* Higher risk of early heart disease
* Greater proclivity toward homosexuality/bisexuality
* More fertile