It was hot at 3 a.m. in a small town in North Carolina, and there wasn’t a lot for a group of teenagers to do. So, Hillary Tillotson, her brother and three other guys sneaked under a fence to go swimming at a private pool down the street. Only Tillotson and her then-boyfriend kept their clothes on, she said.

Two days later, a cop showed up at Tillotson’s house. Some of the teens’ accomplices had been bragging about their skinny-dipping adventure, and someone turned them in for trespassing. She and her brother had to go to court; their mother paid the fine.

“Sometimes I wonder where their brains are at,” Tillotson’s mother, Lori Lee, said of her children. “They do such impulsive things, and sometimes I just don’t think they’re thinking.”

Neuroscientists confirm that teenagers do have brains, but they’re wired differently from those of adults. Why many teenagers seek thrills, break rules and seem nonchalant about their own safety has been a question brain scientists have worked to answer in the last two decades. Top researchers use functional magnetic resonance imaging to see this brain activity.

A new study in the journal Nature found that structural changes in adolescents’ brains correspond to fluctuations in IQ over time, with some young people improving and some falling back on these tests.

Teens improve at such tests at different rates, and it’s difficult to know how someone will do a few years after the initial assessment, said study co-author Cathy Price at the Wellcome Trust Center for Neuroimaging. It’s not yet clear whether fluctuations seen in this study are unique to this age group, or whether they would be similar across a lifespan.

Scientists typically refer to “the teenage brain” in 13- to 17-year-olds, but that doesn’t mean that college students are totally “adults” yet. In fact, research from the National Institutes of Health has shown, the prefrontal cortex, a region of the brain associated with inhibition of risky behavior, doesn’t get fully developed until age 25. The connections between the prefrontal cortex and other areas of the brain are also developing in teenagers. And a number of deep structures in the brain are influenced by changes in hormones, which may lead to heightened emotions.

The way that brain regions talk to one another in teenagers may explain teens’ sometimes confounding behavior, scientists say. Even in their mid-teens, adolescents can make quick, efficient, correct decisions; in the heat of the moment, though, the brain’s deep emotional centers will win out over reason.

“It’s not like these brain parts aren’t there. It’s how they get wired and become fine-tuned with experiences,” said BJ Casey, director of the Sackler Institute at Weill Medical College of Cornell University.

They get what they want

Teens are more sensitive than adults to rewards of situations or activities, and less sensitive to risks, brain imaging research shows.

Laurence Steinberg, a professor of psychology at Temple University and author of “You and Your Adolescent: The Essential Guide for Ages 10 to 25,” has done research in this area. He found that when teenagers are in the presence of friends, the reward system gets aroused even more.

“It’s not that adolescents don’t understand risk. They understand it perfectly well,” says Beatriz Luna, a neuroscientist at the University of Pittsburgh. “It’s just that they find it more rewarding to impress their peers, and things of that sort, than the risk that’s involved to their actual survival; it’s just what they value at that point.”

This would help explain why teens give in to peer pressure more easily than adults — for instance, in breaking the law to jump into a pool in the middle of the night, as Tillotson did.

“I’m not someone who makes big mistakes like that. I’m usually the goody-goody in my family,” says Tillotson, 19, of rural North Carolina.

Her 15-year-old sister, Tessa, is the family’s rebel, their mother says. It’s not unusual for Tessa to sneak out behind her mother’s back and go to the park. “I don’t like my mom to say no,” Tessa says.

There’s an evolutionary explanation for this kind of behavior: In most mammals, adolescence is the time when individuals leave the family environment, Steinberg said. Sensation-seeking leads pubescent creatures to go find sexual partners and a social structure outside the home. They need to become independent of their parents and adapt to new surroundings.

Venturing out into the wild and leaving the security of parents is a risky thing to do, so there must be some built-in biological mechanism to ignore the potential dangers of the wild, scientists reason.

“If it didn’t happen, we wouldn’t leave home and reproduce,” Steinberg explains.

And what they want can be dangerous

The reward-seeking brains of teens may lead them to experiment with pleasure-inducing substances like drugs and alcohol, which are especially dangerous for this age group, scientists say.

Since vital structures in the teen brain are still developing, adolescents are more prone to brain damage from drugs and alcohol. Research has shown that teenagers who binge drink will have greater brain damage than adults. And teens are more vulnerable to stress, which may lead to an increased risk of depression later in life.

Marijuana can stay in a teenager’s system for days, impacting the building blocks of learning and memory. That’s because the teen brain probably has more receptors drugs to bind to — the same is true for alcohol. And in teens who regularly use pot, IQ can permanently decrease, research has shown.

On the plus side, teens are rapid learners, since their brains are still developing.

But that also means they can get “addicted faster, longer and stronger,” Dr. Frances Jensen said at TEDMED in 2010. That’s because addiction is related to learning and memory; teens may be able to pick up a language faster than an adult, but they can also develop dangerous habits more easily, too.

Parenting a teenage brain

So what’s a parent to do? Steinberg says it’s a lot harder to change the reward-seeking predilections of teenagers than to restructure the environment in a safer way.

Parents can make sure kids don’t spend a lot of time unsupervised. For instance, they can enroll teens in healthy after-school programs where adults are present.

On a societal level, raising the price of substances such as alcohol and cigarettes would curb teens’ use of them. This would produce a greater change in behavior among young people than education programs, research has shown.

That’s not to say that kids can’t learn from their mistakes. There’s a broad spectrum of sensation-seeking and tolerance for risk among this age group, so some teens won’t get into as much trouble as others. And even though they may be saying “Get out of my face” when you offer advice, they may be listening and need your support, says Luna, the University of Pittsburgh neuroscientist.

And behavior isn’t entirely determined by biology, either.

Parents can also help by rewarding good behavior instead of emphasizing punishment of negative behavior, Steinberg said. Find an incentive that gets your child motivated — for example, give a bonus in allowance when he or she does something good rather than taking away money as a penalty.

As for Tillotson, after the pool incident, she had to do 40 hours of community service for the chief of police, and she was on probation for six months. Her mother made Tillotson and her brother, who is older and doesn’t live at home, earn money to reimburse her for the fines she paid for them; Tillotson is halfway there.

“I think that’s one of the best decisions parents can do for their children: Make them accountable for their actions,” said her mother, Lori Lee.

A phase that ends with responsibility

As young people move past the teen years and gain more responsibility, they tend to be less inclined toward risk-taking, Luna said.

Tillotson felt guilty about the pool situation, but since then, she’s re-earned her mother’s trust. And she’s making adult choices: going to college, working as a restaurant hostess and postponing her beauty pageant participation to focus on her studies.

She tries to encourage her sister Tessa, who’s been getting into her own share of trouble at home, to think before she acts.

“All teenagers act on their impulse just out of instinct,” Tillotson said. “It’s like ripping off a bandage; you just want to do it quickly, not thinking about it. But what you don’t realize is that you can tear skin. There’s consequences.”