The federal government is embarking on a massive study of young people’s use of marijuana, a project that some say could be a game-changer.

With the goal of following roughly 10,000 young Americans over 10 years, the so-called ABCD (Adolescent Brain Cognitive Development) study will try to answer important questions about how pot impacts young people’s brains, which are still developing into their mid-twenties.

“It’s a great initiative and one that’s likely to change what we know and think about substances, specifically marijuana,” says Dr. Staci Gruber, a Harvard Medical School psychiatry professor and leading marijuana researcher.

The study would start tracking young people at the age of 9 or 10, before they begin ingesting substances such as tobacco, alcohol and pot. Using brain-imaging and other techniques, the $150 million study would attempt to discern differences in those who consumed substances and those who didn’t, or consumed infrequently.

“One of the main questions we’re trying to answer is what these kids look like before they start using substances, and then how much do the substances alter or effect this period of brain development,” said Dr. Susan Weiss, associate director for scientific affairs at the National Institute on Drug Abuse (NIDA), one of the agencies leading the study.

But for all its scope and ambition, can ABCD overcome perceptions of bias and limits on ethical research to provide much in the way of weighty new insights?

Some may believe that decades of research and widespread use have already revealed the risks of marijuana. There’s scientific consensus that most adults can occasionally use pot without serious health risks, and that heavy use by young people is linked with cognitive impairment and psychotic episodes.

But some important findings remain less than conclusive, according to a report by the RAND Corp. for the state of Vermont. The evidence linking pot to stunted brain development, for instance, is “fairly weak and somewhat inconsistent,” the Vermont report says.

Skeptics may point to scientific constraints on ABCD’s findings and value.

Ethics prohibit scientists from giving intoxicating drugs to 9 year-olds. That means ABCD can’t conduct the kind of “gold-standard” double-blind experiments in which subjects are randomly given drugs or placebos. As a study merely observing young people, ABCD can’t show that pot causes anything — good or bad. For all its reach, the study can only establish an association between, let’s say, pot and depression.

That lack of demonstrated causality has left a significant hole in marijuana research. Policy makers are thus relegated to a “fog of uncertainties” about pot’s impact on public health, according to the Vermont report.

Still, some scientists see great potential value in such a large study, which could firm up associations now tenuous because of small sample sizes and other shortcomings in design. ABCD will also take the important step of studying kids before they start consuming substances and studying occasional or infrequent users. “This initiative is absolutely groundbreaking,” Gruber says.

What’s more, the study will use brain-imaging technology such as fMRI, and should offer something decades-old studies haven’t: research on the increased potency of pot and new methods of consuming, such as vaporizing hash oil.

Such a study, Weiss says, has never been more timely. With four states legalizing pot and the movement poised to spread across the country, policymakers need clearer research — particularly on those who appear most vulnerable to pot’s risks — than ever before.

It’s likely that legalizing weed will even be a topic for presidential candidates next year.

That’s precisely what worries some who say marijuana is still caught in a values debate, not a scientific one; a debate that one day will appear inexplicably musty in its archaic views. Federally-funded research has historically focused on the harms of marijuana, especially among very heavy users, instead of its seemingly more moderate risks — even possible benefits — for occasional consumers.

A recent report to Colorado officials noted that pot’s illegality has injected both a “funding bias and publication bias into the body of” research literature on pot use.

Weiss says ABCD won’t be biased. Previous NIDA research may have focused on harm because preventing abuse is the agency’s mission, she says.

Infamous article by America’s first “drug czar,” the man most responsible for prohibition of pot.

But she points to the agency’s interest in research on the therapeutic uses of cannabidiol (CBD), one of the non-intoxicating chemicals in marijuana. ABCD will aim, she said, to answer questions not steer policy. “If it turns out marijuana doesn’t do anything bad to the brain, then that’s just fine,” she said.

The ABCD study involves federal agencies besides NIDA, which could bring different perspectives less focused on harm.

Prohibition is hardly ideal, Weiss, said with its racially-skewed history of enforcement. (A national ACLU study found that African-Americans were four times more likely to be arrested for pot possession than whites, although they consume at roughly equal rates.) “Nobody wants that to happen,” she says.

But the question of legalizing weed — or how best to legalize — is complicated, she says, by questions about pot’s impact on young people’s developing brains.

So researchers are trying to move quickly, by federal standards. The National Institutes of Health announced earlier this month one of ABCD’s first funding opportunities, $2 million for a study coordinating center. Applications are due in April.

Of course, there’s a possibility much of the policy debate may be resolved by the time the study is completed in 10 years. Or, there’s a chance that waiting for its results could be used as an argument against legalizing. “That’s why we’re trying to get it going as quickly as possible,” Weiss says.