Science Watch

For clinicians and researchers alike, cracking the code of why some people are more impulsive than others is likely to yield greater insight into all sorts of human problems, including gambling, violence, substance abuse, social anxiety and other psychiatric disorders--and even physical ills such as cardiovascular disease.

To unravel impulsivity's genetic and environmental origins and how it influences health and behavior, many researchers have looked to nonhuman primates and other animals whose behavioral repertoires are similar to humans', but whose environments and genetic makeup can be more closely observed and controlled.

Such investigations in nonhumans, however, have been hampered by the lack of a satisfactory measure of impulsivity--a dimension of temperament that is broadly defined as the tendency to take risks and to jump into situations without regard for the consequences. Typically, researchers have assessed the trait indirectly. For example, they have observed how animals respond to threatening objects or human handling and searched for evidence of early migration or bite wounds in free-ranging animals. One drawback of such measures, however, is that they may not reflect how animals respond in real social encounters.

In this month's issue of APA's Journal of Comparative Psychology (Vol. 115, No. 1), Lynn A. Fairbanks, PhD, a comparative psychologist at the University of California, Los Angeles, describes a new method for directly assessing social impulsivity in vervet monkeys.

The method--coupled with investigations of the biological and environmental factors associated with impulsivity--opens new avenues for understanding how the trait develops and, eventually, how maladaptive behavior rooted in impulsivity might be curbed, suggests Stephen B. Manuck, PhD, a University of Pittsburgh clinical psychologist who studies biological influences on personality and temperament.

The intruder

Fairbanks sought a test of impulsivity that is reliable, uses direct observations of primate behavior in realistic social encounters, distinguishes impulsivity from related dimensions of temperament, such as anxiety and simple curiosity, and captures the full range of impulsive behavior.

Her solution was an "Intruder Challenge Test," a technique adapted from studies of aggressiveness in rodents. The test measures animals' response to an "intruder"--an unfamiliar animal in a cage at the edge of their territory. Encounters with strangers are common in animals' natural settings, Fairbanks observes, and how they confront such situations is likely to have important consequences for their social status, health and survival.

In her first use of the Intruder Challenge Test, Fairbanks examined 128 adolescent and adult male vervet monkeys living in 16 mixed-sex groups in large, indoor-outdoor enclosures. Three to four monkeys were tested at a time in the 30-minute procedure. During testing, the remaining monkeys in each group were temporarily confined to a separate area.

Observers coded a number of aspects of the monkeys' responses to the intruder, including how quickly--if at all--subjects approached the stranger, behaviors such as touching or sniffing the intruder, and displays of aggression toward the intruder. Because Fairbanks was interested in distinguishing impulsiveness from other dimensions of temperament, observers also noted signs of anxiety, aggressiveness toward other group members and general arousal.

Fairbanks found marked variability in monkeys' behavior. The animals who scored as most impulsive rushed immediately to the intruder, reaching into its cage, sniffing and touching it, and finally settling in for the remainder of the test session.

"Some of these animals were taking a big risk," says Fairbanks. "They ran right over to the stranger before they looked to see what was going to happen. Some of them stuck their hands into the cage--they could easily have had a finger bitten off."

At the other end of the scale were animals who hung back, watching what happened to animals who approached the stranger before making their move. The most avoidant animals never approached the stranger at all. Such avoidant behavior, Fairbanks found, was distinct from anxiety: Many of the animals that were slow to approach the stranger didn't appear anxious, but rather cautious.

In follow-up tests two to nine weeks later, animals' impulsivity scores were similar to the first test, suggesting that the procedure captured a consistent aspect of the monkeys' temperament.

Fairbanks also found that monkeys' impulsivity was associated with their social rank. Two-thirds of alpha males scored in the moderate range of impulsivity, whereas lower-ranking animals were more likely to be more extremely impulsive or avoidant. Fairbanks speculates that animals with a moderate temperament may be best able to build alliances with others, enabling them to achieve high status.

In addition, impulsivity scores were highest during adolescence, when monkeys in the wild usually emigrate from their birth group to a new group. As Fairbanks explains, it is at this age that impulsivity is likely to be most adaptive, helping males confront and compete with unfamiliar adult males in a new group.

Fairbanks has found similar results in a study of impulsivity in female vervets, not yet published.

"This is a dramatic demonstration of the range of how animals respond in social situations," Fairbanks says. "It's not that one personality is good and another is bad. Just as in humans, we're interested in the individual differences and how they create both opportunity and vulnerability to problems. This measure will allow us to understand both monkeys and humans better."

Impulsivity's ancient lineage

Using the Intruder Challenge Test, Fairbanks and others have already begun to try to identify genetic and other physiological links to impulsivity, in hopes of eventually developing more effective ways to address psychological problems in humans, such as social anxiety and extreme social impulsivity.

In an article published in the journal Neuropsychopharmacology in February, Fairbanks and colleagues reported that high impulsivity is associated with low levels of serotonin activity in monkeys' central nervous systems. That study also showed that animals who were given a nine-week course of Prozac, which influences the serotonin system, were significantly less impulsive than animals in a control group.

Recently, Jay R. Kaplan, PhD, a professor of comparative medicine and anthropology at Wake Forest University, has used Fairbanks's method and found a similar association between impulsivity and serotonin activity in female crabeating macaques.

Such findings in nonhuman primates are consistent with an emerging literature examining serotonin's role in impulsivity in people, observes the University of Pittsburgh's Manuck.

"Together, the studies show that there's a common neurobiology underlying this dimension of impulse control, in both humans and old-world monkeys," he says. "It's plausible to infer that the neurobiologic correlates of impulsivity--namely, the variation in brain serotonergic activity--may have an ancient lineage."

In tightly controlled primate colonies such as Fairbanks's, researchers can continuously observe animals' social environments and study the link between genetic relationships and behavior--observations that are virtually impossible to parallel in studies of humans. Now, with a strong measure of primate impulsivity in hand, Manuck says, "Fairbanks is poised to realize the dream of comparative psychology--to elucidate the origins of behavior. And with respect to social impulsivity, she'll be able to do it from the gene, via the environment and through the brain, to the adult social interaction."

This article is part of the Monitor's "Science Watch" series, which reports news from APA's journals.