Terrie Moffitt, a professor in the department of psychology and neuroscience at Duke University, was a co-author of the study, published in the journal Clinical Psychological Science. She said that in the group, the longer people used marijuana, the more their credit ratings fell. "So they were at the point in their late 30s where they wouldn't be able to get a mortgage or borrow money to start a small business," she said. "So this is a pretty crippling level of financial difficulties."
The team also looked closely at people's job performance and relationship issues. Those who were long term, persistent users of marijuana, were more likely to have problems with coworkers and productivity problems, like calling in sick when they weren't actually sick, Moffitt said.
In short, long-term, persistent marijuana use was "not safe" for the users in their study, Cerda said. Problems were comparable to those seen in people who are dependent on alcohol.
"We found that in the case of economic and social harms, cannabis was not safer than alcohol and, in fact, in the case of financial difficulties, cannabis was more harmful than alcohol."
Cerda said she was surprised when they saw that heavy pot users also reported more domestic violence than those who were not heavy users. "We found that it was comparable to the effect of alcohol dependence on abuse," she said.
The people followed are part of the Dunedin Longitudinal Study. Every child born in Dunedin, a city in New Zealand, between April, 1972 and March 1973 was enrolled and has been followed in three to six year intervals since. More than 1,000 papers have been published in the 40 years of follow up about everything from asthma to experience in the workforce. The group is mostly white, but of varying socio-economic levels that is similar to the general population of the U.S., Cerda said.
But critics say the study is inherently flawed, because all current recreational pot smokers in New Zealand -- and in much of the U.S. -- are people who are willing to break the law.
"It's really not a good indicator of marijuana's effects itself," said Mason Tvert, communications director for the Marijuana Policy Project in Washington, D.C., "but more of who's more likely to break the law and use marijuana."
Magdalena Cerda was on KQED Forum Wednesday morning talking about the study, and at least one caller had the same reaction as Tvert. (Others had stronger reactions, you can listen if you want to hear more.)
But journalist David Downs, who writes Legalization Nation for the East Bay Express and described himself as supportive of updating both medicinal and recreational marijuana use laws, said he found the study "super fascinating."
He said the study suggested a common sense approach.
"If you drink a lot, if you smoke a lot, if you do any mind-altering activity heavily every day, I imagine it’s going to alter a lot of people’s life courses," Down said. "I don’t hear anybody saying, 'Smoke pot every day, it’s going to turn out great.' "
(Several people who called or emailed Forum said they were daily users and their lives were fine.)
Professor Wayne Hall directs the Centre for Youth Substance Abuse Research at Australia's University of Queensland. He was not involved with the study, but he said the findings about economic and social problems were consistent with other studies, including in the Netherlands where marijuana is "defecto legal."
"It's an important study," he said and seemed especially interested by the comparison with alcohol use.
"It's telling us what people don't want to hear," he said. "There are patterns with marijuana use that can provide the same social outcomes as patterns we see with drinking."
To be clear, this study did not look at health effects, which the authors agreed are far worse for alcohol than for marijuana. And because alcohol is more widely used it is "still a bigger problem," UC Davis' Cerda said.
But the study comes as it's widely expected that a ballot measure to approve adult use of marijuana will be before California voters in November.
The study authors said they took no position on whether recreational marijuana should be legal. Moffitt said that among the study authors, some used marijuana and some did not. But the study highlights the importance of investing in programs to prevent regular marijuana use and to treat addiction early.
"What we don't know with legalization is if cannabis abuse will start to become more prevalent like alcohol abuse is," said Moffitt. "That's the $64 million question."