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Rintie and Bart, brothers who are 15 and 18, drive recklessly down a narrow, county road on the island of Sjaeland in Denmark. Laughing and listening to loud rock music, they drink beer as they drive. Soon they drive into a muddy farm field. Uninjured, they sleep the night away in their parents’ car.

Last year, Rintie and Bart were hurt when they rode their bicycles, out of control, down a steep hill into an intersection. Both brothers were intoxicated.

On the outskirts of Houston, Keith and Sharon, both 16, are drinking beer, smoking marijuana and driving at 75 m.p.h. A state trooper arrests them.

These teenagers, who live far apart in different countries, were driving without an adult late at night for the first time. Bart and Keith later admitted that they were aware of the danger they created. They both said they “wanted to see what the speed (in their autos) would be like.”

Both of these young, reckless drivers come from intact families with parents who are strict about rules and alcohol abuse. For different reasons, Bart’s and Keith’s parents were unaware that their sons were using their cars on these particular nights.

As risk-taking behavior threatens and ruins the lives of teenagers and innocent bystanders, researchers are finding that the causes and expression of potentially dangerous, teenage actions are complex.

Children who seek out sensational experiences have a higher likelihood of wanting risk-taking.

Like adults, teenagers vary in their levels of sensation-seeking. Children who are high in sensation-seeking enjoy unfamiliarity and higher tension or excitement levels. Low sensation-seeking teens are uncomfortable with novelty and higher tension levels.

Whether and how risk-taking actually takes place is a matter of outside factors, including the legal system, family makeup, peer grouping, school and community traditions.

For example, it turns out that while Keith and Sharon’s dangerous behavior is common in the U.S. among 16- to 18-year-olds, Rintie and Bart’s driving is unusual for Danish teens. Danish children have to wait until 18 to drive, while most U.S. teens can drive by 16 or 17.

By their 18th birthday, 80 percent of American teenagers who drive autos drive at illegal, high speeds. In contrast, only about 10 percent of the same-age Danish teens have high-speed driving records. This increases to 50 percent in boys and 30 percent in girls between 18 and 19.

Similarly, more American teens drive and drink. In Denmark, several studies have shown that as many as 75 percent of 16 or 17 year olds ride bicycles while intoxicated. In Denmark, alcohol can be purchased at age 18. In most U.S. states, the drinking age is 21.

Auto deaths in Denmark among older teens is less than half what it is in the United States.

Thus, in a country where the social norm is toward family permissiveness and relatively fewer parental rules for teens, community consensus, reflected by stricter laws, more powerfully restricts teenagers’ risk-taking.

Parental monitoring-that is, parents’ knowing what their teen is doing-is a more powerful deterrent to teenage risk-taking than strict family rules. If parents don’t know about their teen’s risk-taking, it doesn’t matter how strict they are.

Another area of risk-taking is sexual activity and sexual activity without contraception.

In Denmark, the rate of sex without contraception increases from 20 percent to almost 40 percent as teens age from 16 to 19. Those figures are about the same for American teens. In fact, more than three-quarters of teens in both countries who practiced sex without contraception used contraception at other times. Sex education is apparently not effective in making a difference.

In both countries, sex without contraception, sex with unfamiliar partners, and marijuana use were more frequent in larger cities. This may be because parental awareness of teens behavior is more difficult in larger cities.

For those interested in reading further, the 1972 book, “Youth in Two Worlds: United States and Denmark,” by Kandel and Lesser (Jossy-Bass) is a good starting place.