Where’s the switch in the teen brain that leads someone to addiction, depression and risky behaviors? That’s one of the questions a massive 10-year nationwide study, with some research being done at the University of Utah, aims to answer.

“When we study the person who abuses opiates or abuses marijuana or abuses alcohol, or has major depression or any of the psychiatric issues, the research that has been done is, by and large, after they are in the throes of the illness,” said Deborah Yurgelun-Todd, director of cognitive neuroimaging at the U.’s Diagnostic Neuroimaging Laboratory.

Those studies, she said, present a chicken-and-egg problem: Were the risk factors found in the brain what “led the person to engage in the behavior, or … the consequence of having the disease or treating the disease?”

Scientists hope to watch changes to the brain as such behaviors develop, during the nationwide Adolescent Brain Cognitive Development Study, for which Yurgelun-Todd is the U.’s lead researcher. Utah, Yurgelun-Todd said, has more kids participating in the study than any other site in the nation.

The results could bolster public policy decisions on such issues as whether teens should be tried in court as adults or whether 16-year-olds are mature enough to be issued driver’s licenses.

“What it’s going to do for us is provide a neurobiological basis that’s merged with the social demographic information in a way that’s unprecedented,” Yurgelun-Todd said.

The study’s methodology is the scientific version of the movie “Boyhood,” which traced one kid’s life by filming a few minutes once a year for a dozen years.

The initial recruits are 9- and 10-year-olds, who undergo an MRI scan of their brains, give some biologiclal samples (like saliva), perform cognitive tests and games on a tablet, and take part in an interview. The data from those tests will form a baseline to which researchers can compare data collected later.

Every three or six months, the kids will answer questions by phone, Yurgelun-Todd said, and they will give additinal samples and take cognitive tests once a year. Every two years, they will undergo another MRI.

Researchers are even collecting baby teeth, Yurgelun-Todd said, to get information about early development. The study is working to equip subjects with fitness trackers, to monitor daily exercise habits that are often misreported after the fact.

The study, launched in September 2016, is the largest of its kind ever attempted, with 21 research facilities around the country trying to recruit more than 10,000 children. So far, more than 7,000 youth have been tested.

“There’s never been a sample of 10,000 kids’ brains before,” said Yurgelun-Todd, a nationally known expert in neurobiology who was invited by the National Institute on Drug Abuse (NIDA) to compete to have the U. become one of the study’s sites.

“Some of our specific goals were to see if we can find the neural networks, or neural changes, that put you at risk for engaging in drug use,” she said. More broadly, she added, “You know how people can very frequently know something is not good for them, but they do it anyway? We’re trying to get to the neural basis of that.”

A good example, she said, is determining why someone smokes.

“Is it the social cues of the peer group in school? Is it that everybody in your family is a smoker, so you are, too? Is this something that biologically can be altered?”

Statistical models have been used to draw children from all demographic groups, to provide as broad a spectrum of people as possible. The tests are anonymous, and those processing the data don’t have access to information identifying individual subjects.

Yurgelun-Todd said researchers have gone to science fairs, summer camps and elementary schools to recruit children and parents to take part in the survey. “We’ve really enjoyed how many children really want to know about this,” she said. “They’re curious about what the brain is, and what that has to do with their ability to learn or how well they do in a sport.”

Bryce Hortin, an 11-year-old from Syracuse, signed up because he loves science and wanted to learn more about how his body works.

“My parents tell me I should be a doctor, because when they call something gross that happens on those doctor shows, I say, ‘How is that gross to you? That’s so interesting,’” he said.

Bryce’s mother, Telesa Hortin, was relieved to hear from the research team that his brain was normal and healthy — because she had a brain tumor removed when she was a baby. She said she hopes the study will provide more insight into how the family’s daily living affects his growth.

The study is funded by the National Institutes of Health, backed by NIDA and a raft of federal scientific agencies concerned with cancer, mental health and other issues. Such broad interest, Yurgelun-Todd said, indicates the high expectations of what the data will yield.

The first batch of raw data was released in February, Yurgelun-Todd said, and researchers will take weeks or months to sort through it for various studies. Another batch of data is expected to be released every year for the 10 years of the study.

Nora D. Volkow, director of NIDA, said in a February statement that the initial data showed minimal drug use, as was expected for kids that age.

That finding means the research “will allow us to compare brain images before and after substance abuse begins within individuals who start using, providing needed insight into how experimentation with drugs, alcohol and nicotine affects developing brains,” she said.

One study based on the data dump, Yurgelun-Todd said, is looking at how altitude affects brain development. Utah and Colorado are two of the study sites, so those results can be compared with those from kids in sea-level locations.

“The Utah community has been so wonderful, and engaged, in this study,” she said. “The people here, I think, are invested in the well-being of their children.”

Former Tribune reporter Kelly Schmidt contributed to this article.