Last year, a study came out showing that marijuana legalization in Colorado likely increased cannabis use among adults in the state.

Because of the novel methods the researchers used to examine the question, the study was perhaps the best answer to date on one of legalization’s biggest impacts. But it also left an even bigger question unanswered: Is it bad that more adults are consuming marijuana or doing so more frequently?

Now, in a follow-up study by the same team, using the same methods, the researchers have come to an answer: It doesn’t appear to be.

“At least from the psychological point of view,” said Stephanie Zellers, one of the researchers, “we really didn’t find that the policies (on cannabis legalization) have a lot of negative influence, which I think is important.”

Zellers recently graduated with a Ph.D. in psychology from the University of Minnesota, but she began her doctoral work at the University of Colorado before transferring when her thesis adviser changed jobs. She had originally been interested in neuroscience research, but the necessity of using live lab animals for the work was off-putting to her. And, in the Colorado-to-Minnesota connection, she found a trove of data that could be used in never-before-tried ways.

The data are from longitudinal studies of twins in Colorado or Minnesota. Researchers in both states followed the twins over long periods of time, collecting information about their behaviors, including their cannabis use. The survey information, then, creates an ideal scenario for study: It is thorough, it has built-in controls for variables like educational background or socioeconomic status, and it also accounts better than most for genetic differences.

On top of that, because Colorado has legalized marijuana and Minnesota hasn’t (at least so far) – and because some twins born in Minnesota moved to Colorado as adults and vice versa – the data provide an ideal opportunity to study the way in which a policy change made in Colorado a decade ago has influenced people’s behavior ever since.

“That twin component really allows us to rule out a lot of possible alternatives – maybe there were just cultural differences, family differences, things like that,” Zellers said.

Zellers spoke with The Sun via videoconference from Finland, where she is pursuing postdoctoral research. (And, yes, she is missing sunshine this time of year.)

The original study, published last fall, simply asked whether twins living in legal-marijuana states use marijuana more than twins living in illegal states. And the answer is yes – about 20% more, according to the research.

That answer was interesting, but Zellers said it wasn’t really what the research team wanted to know.

“Really what people care about is: Is legalization harmful,” she said.

To answer that question, the team came up with 23 measures of what they call “psychological dysfunction.” This includes things like substance-use disorders but also financial woes, mental health distress, community disengagement and relationship issues. The team looked at data on more than 4,000 people – 40% of whom live in a legal-marijuana state.

Zellers said what the researchers found was unexpected: They basically found nothing.

“Obviously the cannabis use increases, but we didn’t see an increase in cannabis-use disorder, which is a little surprising,” she said. “We didn’t really see changes in how much people were drinking or using tobacco. No large personality or workplace or IQ differences or anything like that.”

People in legal states did not report using illegal drugs at higher rates. Researchers also didn’t find a link between marijuana legalization and psychotic behavior.

They did find one difference, though. People living in a state where recreational marijuana use is illegal reported higher rates of alcohol-use disorder and more specifically one symptom of the condition: They were more likely to report using alcohol in situations that were dangerous or harmful, such as driving drunk.

To Zellers and other researchers, the study provides valuable information for the ongoing debate over whether cannabis legalization is a good idea. But it’s not the final word.

“Our study suggests that we should not be overly concerned about everyday adult use in a legalized environment, but no drug is risk-free,” CU psychology and neuroscience professor John Hewitt, one of the study’s co-authors, said in a statement. “It would be a mistake to dismiss the risks from higher doses of a drug that is relatively safe in small amounts.”

This highlights one of the study’s big limitations. Zellers said most of the people included in the twins data are relatively light cannabis users. The sample size for heavy users is small.

That means the study can’t tell whether legalization negatively affects heavy cannabis users. It also can’t determine whether legalization is disproportionately harmful for people who may be predisposed to substance-use problems.

“Our sample is an adult community sample broadly characterized by low levels of substance use and psychosocial dysfunction,” the researchers write in their study, which was published this month in the journal Psychological Medicine. “This limits our ability to generalize relationships between legalization, outcomes and risk factors for the individuals at greatest risk.”

For that reason, it is unlikely to settle the debate over whether cannabis is a “gateway drug.” While using marijuana at some point in your life is not indicative that you will go on to use heavier drugs, previous research has found that many people who develop serious drug-use disorders started using drugs by consuming alcohol or cannabis.

Zellers said she and her colleagues are hoping to publish another study based on their data – but this one will be less concerned about the impacts of marijuana legalization as a policy. Instead, it will try to look at how much cannabis people have used over their lifetimes and then score that against the same measures of psychological dysfunction “to see if, not the policy, but the actual substance itself has an effect,” Zellers said.

“We know how people on average live in each state, but that doesn’t tell us about individual people,” she said.