Research showed that the farther certain paired body parts were from symmetry – if one ear, index finger or foot was bigger than another, for example – the more likely it is was that a person would show signs of aggression when provoked. The symmetry effects were different in men and women, however. While the findings may seem strange, there is a plausible explanation, said Zeynep Benderlioglu, co-author of the study and a post-doctoral researcher at Ohio State University.
Deviations from symmetry are thought to reflect stressors during pregnancy – such as poor health, alcohol and tobacco use – that may affect development of the fetus in a variety of ways.
"Paired body parts are presumably controlled by similar genetic instructions, so if everything goes perfectly you would expect paired body parts to be the same size," Benderlioglu said.
"But stressors during pregnancy may lead to asymmetrical body parts. The same stressors will also affect development of the central nervous system, which involves impulse control and aggression," she said. "So while asymmetry doesn't cause aggression, they both seem to be correlated to similar factors during pregnancy."
Benderlioglu conducted the study with Randy Nelson, a professor of psychology and neuroscience, and Paul Sciulli, professor of anthropology, both at Ohio State. Their results were published in a recent issue of the American Journal of Human Biology.
While other studies have examined the connection between asymmetry and aggression, they have come up with conflicting results. This study is different in that it measured aggression in a realistic laboratory setting, and didn't rely on self-report questionnaires. Also, this study, unlike others, used a normal population and not violent offenders.
The study involved 100 college students (51 men and 49 women). Researchers measured differences in size of several paired body parts, including finger length, palm height, wrist diameter, elbow width, ear height and width, foot breadth and ankle circumference. The sum of all the differences in these pairs gave researchers a score of asymmetry for each participant.
The students, who were all volunteers, were told they were going to participate in a study of persuasive ability. They were given a list of phone numbers to call and attempt to raise funds for a fictitious charity organization. But they were actually calling two people involved in the study who were given instructions about how to respond to the study participants.
One of the people the participants called seemed friendly and amenable to giving, but said he didn't have money to donate. But the second charity target was confrontational. He directly challenged the caller and the worthiness of the donation.
The researchers had rigged the phones so they could measure how hard the participants slammed the receiver down after the call – this was a measure of reactive aggression.
Results showed that, in general, the more asymmetry the participants showed in their body parts, the more force they used when hanging up the phone.
But there were also interesting gender differences.
In men, asymmetry was related to a more aggressive response when ending the phone call under the low-provocation condition – when the person simply didn't have money to give. But there was no such association between asymmetry and aggression in the high-provocation condition -- when they talked with the rude charity target.
For women, it was just the opposite – there was no relation between asymmetry and aggression with the low-provocation caller, but women with higher asymmetry scores used more force when hanging up from the rude, high-provocation caller.
Benderlioglu said these results probably have to do with the different ways men and women respond to provocation in general.
"Research has shown that men are quicker to anger than are women," she said. "But while unprovoked men are generally more aggressive than women, the gender differences either disappear under provocation, or women may actually become more aggressive than men."
Studies have shown men are less comfortable then women with high levels of anxiety and other physical changes that occur during conflict.
"So men may be more motivated to quickly withdraw from conflict situations, such as occurred during the high-provocation phone call," she said.
The researchers also looked at the role of testosterone in aggression, and found similar results. Men with high levels of testosterone used more force when slamming down the phone only under the low-provocation condition. In women, higher levels of testosterone were associated with higher aggressiveness only under the high-provocation condition.
The study contained one other twist.
In another measure of aggression, the participants were asked to select one of three follow-up letters to send to the two people who refused to give money. One letter was kind and self-effacing in tone, the second was moderate, and the third was aggressive, saying the people who didn't give money were "performing a disservice to the community."
When they didn't consider asymmetry and testosterone levels, the researchers found both men and women restrained their aggression when choosing letters for the first caller who said he simply didn't have money. In contrast, participants generally selected the harshest letter when responding to the other, more rude charity target.
"Although it is unlikely that anyone would donate any money in response to a letter describing the recipient as a 'disservice to the community,' 33 percent of the respondents chose such letters," she said.
Benderlioglu said the same conditions in pregnancy that lead to asymmetry in body parts probably affects development of parts of the central nervous system that deal with impulse control. The result is that people with higher levels of asymmetry also have a harder time controlling their aggressive impulses.
Other studies have indicated that testosterone is related to a tendency toward anger. So people who show both high levels of asymmetry and high levels of testosterone may react particularly aggressively when provoked, she said.
Nelson said this study was significant in that it measured levels of testosterone at the same time the person was showing aggressive behavior.
"In most human studies of aggression, researchers have gone to prisons and measured levels of testosterone in violent criminals," Nelson said. "But the problem with that approach is that the aggressive behavior occurred years ago. In this study, we were able to measure testosterone at the time the subjects were provoked, which gives better results."
The results emphasize once again the importance of healthy habits during pregnancy, Benderlioglu said. Smoking and heavy alcohol use are among the stressors that may lead to both asymmetry and poor impulse control.
The study was supported in part by grants from the Mershon Center at Ohio State and the National Institutes of Health.
Written by Jeff Grabmeier, 614-292-8457; Grabmeier.1@osu.edu
Journal
American Journal of Human Biology