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In the early 1960s, University of Delaware psychology professor Marvin Zuckerman, PhD, and his fellow researchers noticed something unique about the young men volunteering for their sensory-deprivation experiments: Many were free-spirited types, wearing motorcycle jackets and favoring long hair over the close-cropped style still prevalent in those years. Yet it seemed to Zuckerman, initially at least, that the experiment couldn't have been more dull: Participants lay motionless for hours on an air mattress in a darkened, double-walled soundproof room, the monotony broken only by restroom breaks and cold sandwiches.

Puzzled at the incongruity, Zuckerman then found out what was behind it: Some participants had supposedly experienced hallucinations during prior sensory-deprivation experiments conducted by other scientists, according to newspaper reports. Some of the volunteers now showing up for Zuckerman's experiments came seeking the same hallucinogenic sensations, he says.

He found that these volunteers scored high on a measure he developed to gauge sensation-seeking, and that high sensation-seekers also were more likely to volunteer for experiments on hypnosis and the testing of hallucinogenic drugs.

The discovery helped Zuckerman develop a new sensation-seeking construct for personality, one recognizing the role that an individual's desire for varied, complex, novel and intense stimulation plays in determining personality and behavior. In 1971 in the Journal of Consulting and Clinical Psychology (Vol. 36, No. 2, pages 45-52), he published the Sensation Seeking Scale Form IV, a personality test designed to measure a person's predilection for thrill- and adventure-seeking, experience-seeking, disinhibition and boredom susceptibility.

Subsequent research suggests that high sensation-seeking reaches into every aspect of people's lives, affecting engagement in risky sports, relationship satisfaction before and during marriage, tastes in music, art and entertainment, driving habits, food preferences, job choices and satisfaction, humor, creativity and social attitudes.

Compared with low sensation-seekers, high sensation-seekers are more likely to smoke, abuse alcohol and use drugs, and are more attracted to high-stress careers. Probing further, Zuckerman has found evidence for both a physiological and biochemical basis for the sensation-seeking trait: High sensation-seekers appear to process stimuli differently, both in the brain and in physiological reactions.

High sensation-seekers, who crave novel experiences, are at one end of the scale, while low sensation-seekers, who actively avoid excitement, are at the other end. Most people fall in the middle, with a moderate inclination to seek out new experiences, but a disinclination to push too far, he says.

What's different

When presented with new stimuli, high sensation-seekers have a different orienting reflex (OR) than that of low sensation-seekers. As defined by Zuckerman, the OR is a measure of arousal and interest triggered by any novel object appearing in a perceptual field.

One study found that when subjects with high disinhibition scores were presented with a moderate-intensity tone, their heart-rates slowed down on the first exposure, while the heart rates of low sensation-seekers quickened.

Another of his studies, published in the Journal of Personality (Vol. 58, No. 1, pages 313-345) in 1990, indicates that the differences between high and low sensation-seekers extend to the cortex of the brain, with high sensation-seekers showing an "augmenting" electrochemical reaction, or increasing amplitude of cortical-evoked potentials (EPs) in response to increasing intensities of stimulation. Low sensation-seekers, however, demonstrate a reducing reaction, showing little EP increase in relation to increasing stimulus intensity, and sometimes showing a reduction in EP amplitudes at the highest intensities of stimulation.

The personality trait may have a biochemical basis as well. High sensation-seekers have lower levels of monoamine oxidase (MAO) type B, an enzyme involved in the regulation of neurotransmitters, particularly dopamine, according to Zuckerman's book "Behavioral Expressions and Biosocial Bases of Sensation Seeking" (Cambridge University Press, 1994) and a research review chapter he wrote in the book "Biology of Personality and Individual Differences" (Guilford Press, 2006).

Moreover, research Zuckerman published with M. Neeb in Personality and Individual Differences (Vol. 1, No. 3, pages 197-206) in 1980 determined that sensation-seeking, which is higher in men than in women, peaks in the late teens and early 20s and gradually declines with age, along with levels of testosterone. MAO, which is low in high sensation-seekers, increases with age in the blood and brain.

Since the development of the sensation-seeking scale, Zuckerman has developed the Zuckerman-Kuhlman Personality Inventory measuring impulsive sensation-seeking as a major trait of personality, along with four other major traits: sociability, neuroticism-anxiety, aggression-hostility and activity.

Zuckerman emphasizes that high sensation-seeking is a normal personality trait, despite its association with risky behavior. For example, the trait plays a role in bringing people into prosocial occupations such as law enforcement, firefighting and emergency room medicine-high-stress jobs that would shut down low sensation-seekers.

"In a diverse society, you need both types," he says. "You need people to keep the books and make laws and have families, and you need your adventurers like Columbus to explore and find excitement."

Now a professor emeritus at Delaware, Zuckerman is preparing to publish his third book on sensation-seeking, "Sensation Seeking and Risky Behavior," through APA later this fall.

Looking to the future, Zuckerman says researchers need to learn more about how people's genetic makeup, family environment and social life interact to determine the sensation-seeking aspect of their personalities.

The Big T personality

How a person's thrill-seeking traits fit into the larger society-and how society can channel positive aspects of thrill-seeking and dampen negative aspects-is a question that fascinates Temple University psychologist Frank Farley, PhD. A former APA president, Farley has developed a personality model that describes the Big T (thrill-seeking) personality.

"To me, one of the deepest motivations in the human spirit is to lead an exciting, interesting and thrilling life. It's not for everybody, but it's a powerful force," he says.

Farley's study of thrill-seeking has taken him to Nepal, where he interviewed Mount Everest climbers, to China and later to the Baltic states where he participated as a crew member in cross-country hot air balloon racing. He travels the world seeking extreme risk-takers, who provide him, he argues, a more valid profile than college students do.

"If I want to study major risk-taking, I've got to go to where the major risk-takers are," he says.

In Farley's model, the Big T "positive" personality can account for involvement in entrepreneurship, extreme sports such as parachuting and hang-gliding, or creative science and art. By contrast, the Big T "negative" personality may turn to crime, violence or terrorism "for the thrill of it"-embracing the destructive, dark side of the trait.

A Big T positive personality can find thrills in physical or mental activities. Albert Einstein, for example, was a Big T "mental" personality who found intellectual discovery thrilling. Farley sees thrill-seeking everywhere, from special effects-laden Hollywood blockbusters to the hundreds of thousands of fans who gather annually at the Indianapolis 500 motor race, enjoying the vicarious thrill of watching hurtling race cars (see "Psychologists are thrill-seekers too"). It extends to the highest reaches of creativity and innovation in science, business and education, as he outlined in a chapter in "Fostering Creativity in Children, K-8: Theory and Practice" (Allyn and Bacon, 2001).

Farley theorizes that in a country such as the United States-a Big T nation built on the risky adventure of immigration-thrill-seekers are given more freedom to pursue their quest for bigger thrills and risks than in countries with more structured cultures, such as China.

Often democratic societies benefit economically, as risk-takers become ever more creative in their endeavors, says Farley. He cites as an example of a creative risk-taker Microsoft founder Bill Gates-a college dropout whose ideas helped revolutionize how society uses computers.

Despite America's tradition of thrill-seekers, Farley sees a constant tension between thrill-seekers and people who want stricter safety regulations, citing the ongoing debate over the toll of climbers killed on Mount Everest every year. Mountain climbers think the chance to reach the top is worth it, despite the risk of dying.

"Their view is, 'We're all going to die. I'd rather die undertaking a grand adventure than in bed with tubes running through my body,'" he says.

A sense of calm

While psychologists like Farley research thrill-seekers, others like Chris Carr PhD, focus their practice on them. Carr works with athletes who might be considered sensation-seekers and thrill-seekers by anyone not involved in their sports.

An Indianapolis-based sport psychologist, Carr served as team psychologist for the U.S. men's alpine skiing team from 1992 to 2002 and is currently working with the U.S. national diving team. He also consults with Rising Star Driver Development, a Chicago-based firm that helps younger race car drivers transition into professional racing. The skiers he's worked with rocket down icy slopes at speeds topping 70 miles per hour, while the divers leap off platforms more than 32 feet above a pool, twisting and turning to the water below.

Interestingly to him, the elite athletes he works with don't talk so much about the thrill of pulling off such physically challenging feats, but rather about the sense of calm they feel when performing at their peak, Carr says.

"I think they love the sensation of moving; they love the sensation of being in control when maybe everyone else would feel out of control," he says.