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Ed Yeates reports

We all know people, or know OF people, who just have to take a risk -- people who skydive and bungee jump and run wild rivers.

It turns out that these people may not be seeking thrills, just for the thrill of it.

There's scientific research that says they may not be able to help themselves.

Science Specialist Ed Yeates reports from a popular site for people who live on the edge.

Pick any amusement park in the country, like our own Lagoon here, and you'll find thrill rides designed to give people that so-called adrenalin rush... to push the envelope of the so-called "fearless factor."

Layne Rutschke and Jamie Hansen are strapped into a capsule called "The Catapult."

It sits there for awhile, building up tension - then shoots them 250 feet into the air from zero to 90 miles per hour in three and a half seconds.

The thrill is short-lived.

ED YEATES: "THE ADRENALINE RUSH ON THIS COASTER TAKES ONLY ABOUT 35 SECONDS. BUT THEORETICALLY - FOR SOMEONE BORN WITH A GENETIC PREDISPOSITION FOR THRILL SEEKING - THE RIDE GOES ON FOREVER."

Issac Wilson in Park City is on one of those lifetime rides. And it's been that way since childhood.

ISSAC WILSON, ECO-CHALLENGE ADVENTURER: "I CRAVED IT. YAH. IT WAS ONE OF THOSE THINGS WHERE MY DAD WOULD PULL OUT THE MAPS AND LAY THEM ON THIS TABLE AND SWISSSH - YOU WOULD INSTANTLY GO OVER AND JUST START LOOKING AT IT AND THINKING ABOUT THE NEXT ADVENTURE."

"FOR ME, IT'S JUST ONE OF THOSE THINGS - YOU JUST - IT'S WHAT I HAD TO DO."

And what adventures to do!

Issac bikes, hikes, and runs up and down rock mountains - kayaks in the world's most rugged waters - and sails San Pan canoes in the Philippines.

He races on camels in Morocco, Arabian stallions in North Africa - and sky dives out of planes in Madagascar.

WILSON: "THE YEAR BEFORE IN BORNEO WAS JUNGLE TRACKING, CAVING, OUTRIVER CANOEING, SCUBA DIVING - I MEAN EVERY SPORT YOU CAN POSSIBLY IMAGINE."

Issac competes in what are called World Eco-Challenge events. They attract only "gutsy" competitors who researchers believe may be genetically enticed to do what they do.

DEAN HAMER, NATIONAL INSTITUTES OF HEALTH: "IT IS JUST THIS REAL DESIRE FOR STIMULI. IT'S A REAL SORT OF SUSCEPTIBILITY TO BOREDOM IF YOU WILL AND IT CAN BE GOOD OR BAD DEPENDING ON WHAT THE STIMULI ARE."

Dr. Dean Hamer says there is strong evidence now that genes are involved in normal variations which make us unique.

One person is adventurous and fearless while another is shy and cautious. But both traits are NORMAL.

HAMER: "THEY MAY BE SHY NOT BECAUSE THEY'RE TRYING TO GET AWAY FROM OTHER PEOPLE BUT THAT'S JUST THE WAY THEY ARE BORN. AND ANOTHER CHILD MAY BE VERY ADVENTUROUS - NOT BECAUSE THEY WANT TO BE BAD BY GOING AND POKING AROUND WHERE THEY SHOULDN'T BE - BUT THAT'S JUST IN THEIR GENES."

In Issac Wilson's life - he seeks the extreme - and likes it.

In this shed full of equipment he's added specialized picks this year so he can experience ice climbing for the first time.

Issac describes kayaking alone in Scotland from the southern most island of Barrow to the northern most island in the Isle of Harris - in freezing temperatures and a storm.

ISSAC: "THESE 25 FOOT SWELLS IN A VERY CONFUSED SEA WHERE IT'S KIND OF BASHING AROUND AND YOU'RE BOBBING AROUND LIKE A CORK IN A LITTLE KAYAK - AND IT'S BASICALLY YOU AND THE OCEAN WITH JUST SUB-FREEZING BASICALLY. YOU FALL OVER - YOU DIE, YOU KNOW. I WAS ON A BUZZ FOR WEEKS AFTER THAT."

Issac says he continually craves the stimulus - the challenge. And as soon as he gets home, he's immediately planning the next trip.