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For instance, "parents appear to play a critical role in the development of the serotonin system," which occurs during childhood, Higley says. In monkeys, association with nurturing parents can lastingly curb arousal and anxiety in their offspring. The impact becomes apparent when comparing monkeys raised by their mothers to "peer-reared" monkeys raised in the absence of adults. Serotonin system development of the monkeys in the two groups begins to diverge as early as two weeks of age–with the peer-reared monkeys showing diminished levels of 5-HIAA–and the differences persist well into adulthood. THOSE RESULTS show the impact of the loss of good parenting on young monkeys with normal genes. Higley and his NIAAA colleague Allyson J. Bennett performed another study that revealed the impact of good parenting on young monkeys with defects in the genes that determine the characteristics of the serotonin reuptake system [Mol. Psychiat., 7, 118 (2002)]. A short allele produces an impaired transporter for the serotonin reuptake system that can lower the amount of available serotonin. Higley and Bennett found that rearing by a mother compensates for the serotonin system deficit caused by this genetic defect. Regardless of whether they had the short or long version of the allele, mother-reared monkeys produced about the same amount of 5-HIAA. The buffering effect is lost when monkeys' parents are missing and they are raised by peers. Peer-reared monkeys with the defective allele produced much lower levels of 5-HIAA than those with the regular allele. Higley is trying to find out how rearing by a mother overcomes the transporter defect. Apparently, environment can compensate for poor genes. In some cases, the reverse is true: Genetics can confer protection from a hostile environment. Avshalom Caspi, a University of Wisconsin psychology professor, published a paper last summer about the interaction between genes and the environment [Science, 297, 851 (2002)]. Caspi and his colleagues reported that they "studied a large sample of male children from birth to adulthood to determine why some children who are maltreated grow up to develop antisocial behavior, whereas others do not. The lead-exposed monkeys proved to be so aggressive that the test was halted early because an animal was injured. Specifically, the researchers looked at genotypes that affect production of monoamine oxidase A (MAOA). This enzyme breaks down dopamine and norepinephrine–two neurotransmitters that may promote aggression–as well as serotonin. Depressed levels of MAOA enzyme are associated with aggressive behavior. The team found that "maltreated children with a genotype conferring high levels of MAOA expression were less likely to develop antisocial problems" than those with low MAOA expression. Generally speaking, however, kids who are severely punished or who witness aggression or parental dysfunction are more likely to exhibit aggressive behavior later in life, Coccaro says. One hypothesis suggests that people with this kind of history develop abnormalities in their social information processing skills. For instance, if a participant in a karate match hurts an opponent in the process of winning the match, the loser could interpret the situation in any number of ways. "The average person who doesn't have these sorts of deficits in social information processing and hostile attributional bias–which is what you tend to get when you have been abused as a child–would say, 'It was an accident,' or, 'That's what he thought he needed to do to win the match,' " Coccaro explains. "All things being equal, if you think somebody did something to you by accident, you're not going to retaliate." On the other hand, "Someone who's been abused as a child and who's experienced a lot of aggression will say, 'That guy wanted to hurt me. He wanted me to look bad.' " In turn, Coccaro says, "that's going to be a trigger for him to do something back. What's underneath that neurobiologically, we're not certain of. But there are brain circuits involved in emotional regulation processing, and they're probably affected."
In fact, positron emission tomography (PET)and magnetic resonance imaging scans indicate that the prefrontal cortex is smaller or less active in impulsive murderers and those who have antisocial personality disorder (ASPD) than in control subjects and "cold-blooded" killers who plan their crimes, according to University of Southern California psychology professor Adrian Raine. The prefrontal cortex, the outer part of the brain located just behind the eyes, controls impulses and is involved in emotion, arousal, attention, the interpretation of sensory stimuli, and conscience. Raine has also determined that both types of murderers have higher activity in the right-hemisphere subcortex than control subjects. The subcortex includes the amygdala, hippocampus, midbrain areas, and thalamus. It is believed to be involved in generation of aggressive feelings and behavior, Raine notes. In addition to parenting quality, other environmental influences include television and other media that can deliver violent content to minors. Results from research into the behavioral impact of such viewing have for the most part been contradictory, according to Coccaro. But he is impressed by data recently published by L. Rowell Huesmann, who tracked the TV viewing habits of kids as they grew up and correlated it with the development of aggressive behavior [Dev. Psych., 39, 201 (2003)]. Huesmann, who heads the University of Michigan's Aggression Research Program, is a professor of communication studies and psychology.
More tangible environmental hazards include lead. While working at the University of Wisconsin's Harlow Center for Biological Psychology, senior scientist Nellie K. Laughlin studied a group of adult female monkeys that had been exposed to lead for research into the metal's impact on babies. Laughlin observed the social behavior of small groups of the adults, including activities such as grooming. "I was naive," she recalls. "I thought, 'They're females; there won't be any aggression.'" But Laughlin was wrong. The lead-exposed monkeys proved to be so aggressive that the test was halted early because an animal was injured. In a recent paper, Herbert L. Needleman, a professor of child psychiatry and pediatrics at the University of Pittsburgh School of Medicine, and his colleagues confirmed the link between delinquency and elevated body burdens of the metal [Neurotoxicol. Teratol., 24, 711 (2002)]. Lead interferes with synapse formation, lowers serotonin levels, and increases dopamine sensitivity, among other central nervous system effects, he wrote. And it may indirectly increase the likelihood of misbehavior by impairing cognitive function and school performance. However, "the specific biological mechanisms underlying lead's effect on aggression and impulsiveness are not known," according to Needleman. Other factors that have been blamed for inciting aggression include poor nutrition. Deficits in consumption of vitamins, minerals, and essential fatty acids may interfere with the production of neurotransmitters. Oxford physiologist C. Bernard Gesch found that administering nutritional supplements to young adult prisoners significantly reduced their antisocial behavior, including violence, as measured by disciplinary offenses [Br. J. Psychiat., 181, 22 (2002)]. Some researchers believe that low levels of cholesterol may be linked to an increased likelihood of violent behavior and violent death, possibly resulting from reduced serotonin activity. Backers of this controversial theory include Beatrice A. Golomb, a physician at the Veterans Affairs Medical Center in San Diego. One risk factor about which there is no controversy is alcohol, considered to be "one of the largest contributors to violence," Higley says. "About half of the episodes that occur–whether they are murders, rapes, or whatever–tend to be under the influence of alcohol."
ALCOHOL MAY "lead a person to misjudge social cues, thereby overreacting to a perceived threat," according to a statement from NIAAA. Furthermore, alcohol may "lead to an inaccurate assessment of the future risks of acting on an immediate violent impulse." From a physiological point of view, alcohol may exacerbate aggressive tendencies by metabolizing serotonin. In addition, ASPD, which is characterized by a "disregard for the rights of others, often manifested as a violent or criminal lifestyle," may share a genetic basis with early-onset alcoholism, according to NIAAA. Higley's low 5-HIAA monkeys drink more alcohol and have a harder time stopping when they start. Once they're drunk, they are more likely to be aggressive than monkeys whose serotonin systems function normally. Higley adds: "Alcohol appears to lower the threshold that it takes to elicit aggression, making individuals that are already aggressive especially dangerous. In fact, the only time that I've ever been chased by any of these monkeys–which are about a sixth of my size–is when they are intoxicated. And the ones that do it are the animals with low serotonin." Illicit drugs, including cocaine and phencyclidine (known as PCP or angel dust), are also associated with increased violence. Cocaine damps down serotonin synthesis and release and boosts release of excitatory neurotransmitters such as adrenalin. It also interferes with the reabsorption of dopamine. PCP, too, affects multiple neurotransmitter systems. Researchers are clearly making headway in their studies of violence and aggression. But their work yields complicated legal and ethical conundrums. For instance, it seems logical to punish a violent criminal. But given what scientists have learned about the serotonin system, can a person who commits a violent act as a result of flawed brain chemistry be held responsible for the deed? "People who have risk factors for being more likely to commit an aggressive act should get treated for those factors if possible," Coccaro says. "But it doesn't free them of criminal responsibility. They can still step back from these behaviors." The mitigating factors of biology and life history are more relevant to sentencing, he believes. Another equally thorny issue concerns prophylactic intervention. "In our society, we would not espouse treating individuals simply because they have some sort of characteristic that makes them more prone to developing aggressive or violent behavior," Higley says. Nevertheless, he is analyzing whether early behaviors can serve as warning signs that a child may develop into a violent or aggressive person. In monkeys, one revealing type of encounter is rough play, which young animals use to learn to express aggression at an intensity that's appropriate to the setting. "It appears that early play behavior predicts which individuals are going to become aggressive later on, with monkeys that practice rough play a lot early in life learning to inhibit aggression and express it appropriately," Higley concludes from his studies. "Monkeys that seldom play early in life, on the other hand, do not learn such skills, and they act aggressively when they become teenagers." Even if a particular warning sign is present, however, it may not prove a definitive signal. "It's pretty clear that biology is not destiny," Higley notes. "It's a risk, but it's not a guarantee." Page: 1 | 2 Chemical & Engineering News |
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