Put two people at the top of the same cliff. One buckles a parachute, laughs, and steps off into a thousand feet of air. The other watches from the edge with a knot in the stomach, works out the odds, and drives home to a quiet evening. Same drop, same physics, two entirely different animals. For sixty years psychologists have been trying to say, precisely, what makes them different — and the answer has led out of the questionnaire and into the chemistry of the brain.
The man who set out to bore people
The trait got its name from a researcher who was trying to do the opposite of thrill. In the late 1950s and early 1960s Marvin Zuckerman was studying sensory deprivation: he put volunteers in dark, silent isolation chambers and watched what the absence of stimulation did to them. He expected the experience to be uniformly unpleasant. It wasn’t. Some people found the emptiness intolerable within minutes and clawed to get out. Others sank into it, calm, even content, and a few seemed to want more of it than the study could give.
The reactions didn’t line up with any personality measure Zuckerman had on hand. So he built one. In 1964, a decade after his doctorate and at the start of the work that would occupy him until his death in 2018, he published the first Sensation Seeking Scale — a questionnaire designed to sort people by how much novelty, intensity, and risk they needed to feel right. It eventually settled into four flavors: thrill and adventure seeking (the parachute), disinhibition (the wild party), experience seeking (the one-way ticket to a country you can’t pronounce), and boredom susceptibility (the person who cannot sit still through a slow afternoon).
Psychology had long held that everyone has an optimal level of stimulation — the idea runs back to Wundt. Zuckerman’s addition was that the level isn’t the same for everyone. A high sensation seeker walks around chronically under-stimulated; ordinary life feels flat, and it takes something loud — speed, danger, a new city, a fight — to bring the nervous system up to where it feels normal. The cautious person is already there. To them, the skydiver’s normal is an emergency.
Born, not just made
The first hint that this was biology and not merely temperament came from twins. When researchers compared identical twins to fraternal ones, sensation seeking turned out to be substantially heritable — estimates cluster near 60 percent, at the high end of the ordinary range for a personality trait. The more telling result is that the finding held for identical twins raised apart in different homes — one such study put it at 59 percent, essentially the same as for twins raised together, which undercuts the easy explanation that thrill-seekers simply grow up in thrill-seeking families.
Something measurable, then, was being handed down. Zuckerman went looking for it in the blood and found a candidate in an enzyme called monoamine oxidase, or MAO — the cleanup crew that breaks down dopamine, norepinephrine, and serotonin after they’ve done their work in the brain. People who scored high on his scale tended to have low platelet MAO. The correlation is modest — a 2025 meta-analysis of fourteen studies and some 1,470 people puts it at −0.22 — and the blood measure is shakier than it sounds: platelets carry mainly the B form of the enzyme, and the same meta-analysis notes that MAO B activity in the cortex doesn’t actually track MAO B in the platelets. Whatever the blood is reporting, it is not a readout of the brain. It was still enough to point Zuckerman somewhere specific. Less of the enzyme that clears these chemicals away, Zuckerman reasoned, means they linger longer and hit harder. In one of the more on-the-nose results, sixteen professional bullfighters were found to have significantly reduced platelet MAO activity against forty-six controls — and to score, unsurprisingly, as more extraverted and sensation-seeking than the men who don’t step in front of the bull.
The plumbing under the appetite
The clearest picture of what’s happening came in 2008, from David Zald and his colleagues at Vanderbilt. They took 34 healthy adults, most of them in their twenties, scored them on Cloninger’s novelty-seeking measure, and put them in a PET scanner to look at a very particular piece of the dopamine system: the autoreceptors of the midbrain, where dopamine cells originate. Thirty-three of the scans came through clean enough to use.
Autoreceptors are the brain’s own brakes on dopamine. They sit on the dopamine-producing cells and sense how much has been released; when levels rise, they tell the cell to ease off. More autoreceptors, tighter regulation — the tap gets shut sooner. Fewer autoreceptors, and the dopamine keeps coming.
The people who scored highest on novelty-seeking had the fewest autoreceptors in the midbrain. The relationship was inverse and strong — a correlation of −0.68 at its peak, still −0.64 after accounting for age: the stronger the appetite for the new, the weaker the built-in brake. Zald’s framing was that dopamine is the fuel and the autoreceptors are the regulator, and that in the sensation seeker the regulator is simply lighter. Give a brain a strong reward signal and a poor ability to hold it back, and you get a person who pushes limits — because the reward for pushing them lands harder in that brain than it would in yours.
This is the counterintuitive part, and it’s worth sitting with. The finding is not that the daredevil’s brain is simply swimming in more dopamine. What Zald’s group measured was the brake, not the fuel, and what they predicted from it was a difference in the response: when something rewarding or novel shows up, the dopamine surges with less to check it. The term Zald’s paper reaches for is an “exploration bonus,” borrowed from the modelers who named it — the new thing pays out more in that brain than it does in yours, so the brain goes and gets more of it. Whether the same wiring also leaves ordinary, unstimulating life feeling flatter is the older idea Zuckerman argued for, and it fits, but it is not what the scanner showed.
The other end of the scanner
Caution has its own machinery, and it isn’t merely the absence of the daredevil’s. When Martin Paulus and colleagues put people through risky choices in a scanner in 2003, one region tracked caution: the right anterior insula, a fold of cortex that registers the body’s internal states — the churn of dread, the somatic flinch. It is the same area that fires with disgust and with anticipated pain. The harder it worked during a risky decision, the higher that person scored on harm avoidance — and the more likely they were to retreat to the safe option after being burned once.
The cliff-edge accountant, in other words, isn’t failing to feel the pull of the jump. A different circuit is speaking louder — one that turns the abstract idea of a bad outcome into a felt, visceral aversion, and does it fast enough to keep the feet planted. Cloninger, another architect of this field, mapped these tendencies onto separate chemical systems: novelty seeking riding on dopamine, harm avoidance riding on serotonin. The bold and the careful aren’t opposite ends of one dial. They’re two dials, two systems, that happen to be set differently in different people.
The trait that walked out of Africa
If a loose dopamine brake really does push people toward the new and the far, you might expect its genetic signature to show up wherever humans went looking for new and far places. There is a gene, DRD4, that codes for a dopamine receptor, and one common variant of it — the version with seven repeats of a small DNA sequence, the 7R allele — turns up again and again in studies of novelty-seeking and financial risk-taking.
In 1999 a team led by Chuansheng Chen pulled together DRD4 data from 2,320 people across 39 populations and set it against how far each group’s ancestors had migrated. The pattern was hard to miss. The farther a population had traveled out of Africa over the last tens of thousands of years, the more common the long, restless alleles. The correlation with long-distance migration ran to 0.85 — startlingly high for anything in behavioral genetics.
The obvious objection is that populations that traveled far are also populations that drifted apart genetically, and drift alone could produce that pattern with no selection involved. In 2011 Luke Matthews and Paul Butler went back and modeled exactly that, controlling for neutral population structure, and found the association with the 2R and 7R alleles survived it. It still doesn’t prove the gene drove the journeys, and the causation is argued over. But there it sits: the version of the receptor tied to itching for the new is most concentrated in the descendants of the people who kept walking.
What the setting is for
None of this is destiny, and the trait is not a verdict on character. The same loose regulation that produces the reckless gambler produces the surgeon who stays calm in a crisis and the explorer who volunteers for the trip no one else will take; the same tight regulation that keeps a person off the cliff keeps the books balanced and the bridge standing. Sensation seeking peaks in the late teens — a study of more than five thousand people across eleven countries put the summit at nineteen — and fades across adulthood, tracking a dopamine system whose receptor counts also fall decade by decade. Which is one reason the wild are so often young and the careful so often gray.
The skydiver and the accountant, then, are not making a free choice about how much risk to court, or not only that. They are each running a nervous system tuned to a different level, reaching for the same feeling of being right, and finding it in opposite places — one at the edge of the cliff, one at a safe distance from it, both convinced the other has lost their mind.