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Born to Be a Bully?: Study shows social stress can change your mind

Tacked up beside Yvon Delville’s computer is a microphotograph of a stressed brain. Swollen clusters of black dots on the rumpled photocopy tell viewers all they need to know about nature versus nurture: your environment, literally, can change your mind.

Dr. Yvon Delville holding a hamster
Dr. Yvon Delville's research with hamsters is providing answers to the source of aggressive behavior in humans.

For the past decade Delville, a University of Texas at Austin psychology professor, has conducted research that yields intriguing insights into aggression, social stress and brain biochemistry during a child’s development. Delville’s research has evolved right alongside a spate of media-hyped acts of violence by teenagers over the past few years.

Very curious about the brain mechanisms that control aggression and that may change because of exposure to threat and stress, Delville began to examine the effects of social stress and aggression on a rather late developmental period: adolescence.

Because of their solitary and territorial predisposition, Delville selected golden hamsters as the animal model with which he would experiment. Hamsters are weaned at around 25 days of age, at which time the mother bumps them from the nest, and they venture out to live on their own. At this age they can be described as hamster teens.

In a series of experiments, weaned, male pubescent golden hamsters were placed for an hour a day, for two weeks, into an adult hamster’s cage. The older hamsters’ territorial and antisocial tendencies compelled them to respond with aggression and hostility to the adolescent males, nipping and chasing them. Although the adult hamsters’ behavior was antagonistic and disturbing to the young hamsters, it was not violent or lethal.

A second control group of adolescent male hamsters were simply placed in an empty, unfamiliar cage for one hour a day. Both environments elicited stress reactions in the young hamsters, but the long-term effects of the two different kinds of stress were surprisingly dissimilar.

Cortisol, a stress hormone secreted by the adrenal glands in hamsters and humans, was found to be high in both groups of adolescents during the first day of the experiment. However, the cortisol levels remained elevated for the entire two weeks only in the chased and threatened hamsters. The bullying was a stressor to which they could not seem to adapt.

“The concern with humans is that children who are bathing their brains in cortisol may be at risk for abnormal brain development and irreversible changes to the brain,” said Dr. George Holden, associate chair of The University of Texas at Austin Department of Psychology.

Two adolescent hamsters engaging in benign play behavior
Before they are repeatedly exposed to threat and stress, adolescent hamsters engage in benign play behavior.

Closely observing the young males that had been threatened by hostile adults during the experiment, Delville and graduate students Joel Wommack and Kereshmeh Taravosh-Lahn saw that the traumatized adolescents exhibited exaggerated attack behavior toward smaller males, while being fearful and subordinate with hamsters of equal size or larger. The kitten-like play behavior of their youth had disappeared.

The bullied hamsters had turned into classic bullies themselves. Both their behavior and their brain chemistry revealed a distinct transformation.

“What was found was that the stressors in the environment accelerate the onset of adult-like behavior and the termination of childhood play-fighting,” Delville said. “The exposure to stress and threat make the adolescents attack earlier and act more like adults.”

In contrast, control littermates who had spent time alone in a novel environment showed no lasting changes in behavior.

An examination of the bullied adolescent hamsters’ brains showed significant changes in the concentrations of the neurotransmitters vasopressin and serotonin. In humans, high levels of vasopressin are associated with increased aggression, while serotonin is known to inhibit aggression in males.

The levels of vasopressin in the hypothalamus were lower in abused hamsters than in their littermates in the control set, and levels of serotonin were higher. These neurotransmitter levels correlated well with the cowardly behavior that the stressed hamsters showed and suggested that the chemical makeup of a bully may be unique.

In addition to vasopressin and serotonin, another neurotransmitter called dopamine had also been altered. Visible on microphotographs of neurons was graphic evidence that the expression of dopamine increased when the hamsters were bullied, and this increase occurred in the amygdala, a social integration center in the brain that is responsible for aggression and social interaction.

Most important, the chemical cocktail of the distressed hamsters’ brains seemed to point to the fact that adolescence is a singularly significant period in the development of adult social behavior. Traumatic experiences and the threat of stress and attack early in life affect the topography of the brain as well as behavior.

“Scientists first thought that most brain development occurred in the first four years of life, but Dr. Delville’s research shows that this later time is a highly sensitive and crucial period of development,” said Holden. “Not many scholars are doing studies of bullying, and Delville is one of the only ones who’s successfully linking the behavior with neurochemistry.”

In the past year Delville has advanced his research into bullying and aggression a step further and has begun, with colleagues, to test human subjects.

For two semesters Delville administered questionnaires to a mix of male and female freshman psychology students, trying to determine if repeated exposure to social and environmental stressors during adolescence alters one’s concept of inappropriate and appropriate levels of violence. Delville hypothesized that students who reported having been bullied would, perhaps, view lethal violence more as a norm than an abnormal method of avenging harassment and insult.

Microphotographs of the brains of a bullied hamster and a hamster not exposed to bullying
The brain of a bullied hamster (left) shows an increase in the expression of the neurotransmitter dopamine.

The questionnaires presented several scenarios in which teenage males or females are publicly humiliated, hurt, irritated or ridiculed. In some scenarios, the offended teen is bullied and then reacts in an extreme fashion to resolve the conflict. In other scenarios the subject is simply presented with the stress and aggravation of a crashing computer or a driver who cuts him or her off in traffic.

As predicted, students who participated in the study and who reported having been bullied, verbally abused, isolated and unpopular in high school were more likely to view the use of a gun, for example, as a reasonable means of settling a score. They were prone to sympathize with the violent, often lethal, actions of bullied victims in the questionnaires' scenarios.

Female students who reported having been bullied were more likely to indicate that they suffered from several markers of depression—feelings of inferiority, isolation, loneliness, anxiety, sadness. They were also more likely than their non-bullied female counterparts to express passive-aggressive desires for revenge and retaliation.

Both males and females who reported being bullied indicated that 15- and 18-year-old males engaged “in a serious fight” are very likely to use guns and violence to settle the dispute. These questionnaire responses seem to indicate that repeated exposure to aggression and threat during adolescence may forever alter concepts of violence in adolescents and lead them to view lethal violence, for example, as acceptable.

The results of these pilot studies on human subjects mirror Delville’s findings with animal models. Like the hamsters, humans seem to experience an accelerated development of antagonistic behavior if they have been exposed to ongoing stress at certain periods in their development. The effects of environmental and social stressors during an isolated period of development have the potential to be lasting.

“What, to these adolescents, is acceptable versus unacceptable violence? The kids’ perceptions of that is where bullying comes in and how you may get teenagers carrying guns and shooting at other students in their schools,” said Delville. “Looking at nature versus nurture, I think that each plays an equally important role. It’s amazing, but these experiments show that our brain actually changes because of events in our environment.”

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  Updated 2014 October 13
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