In the debate over what marijuana legalization means for Colorado, the best drug-policy brains in the nation say there is one question getting short shrift: If people can more easily toke, does that mean they will drink less?

It is, for now, a question without an answer. But what that answer is, the experts say, will be a big factor in determining whether marijuana legalization is worth it.

“A small change in alcohol has a bigger social impact than a large change in cannabis,” said UCLA professor Mark A.R. Kleiman, one of the authors of the book “Marijuana Legalization: What Everyone Needs to Know.” “So it ought to matter a lot whether the change is in the right direction or wrong direction.”

The legalization of use and possession of small amounts of marijuana for those 21 and older in Colorado and Washington will provide researchers worldwide with the best chance ever to study the interplay of alcohol use and marijuana use. The topic is what academics refer to as “cross-price elasticities of demand.” It basically means how changes in the price and accessibility of one substance impact the use of a different substance.

What researchers will be trying to settle is whether alcohol and marijuana are substitutes — meaning people use one but not the other — or whether they are complements — meaning they are used together.

If they are substitutes and people switch from booze to pot, experts such as Beau Kilmer, the co-director of the RAND Drug Policy Research Center, say marijuana legalization could be a net benefit for society, even if problems related to marijuana use increase.

“The social costs of marijuana use really pale in comparison to what we see as the social costs of heavy alcohol consumption,” Kilmer said.

He and Kleiman both point to widespread alcohol-fueled violence and disease as reasons why a slight downtick in alcohol abuse could outweigh a large uptick in marijuana abuse.

But, if marijuana and alcohol prove to be complements, the problems of both would only be worsened. People driving both drunk and stoned, for example, are more dangerous than drivers impaired by only one of the substances.

“What’s really potentially harmful is when people are using alcohol and marijuana together at the same time,” Kilmer said.

Researchers have never been able to get a clear picture of the relationship because, until November, no place had legalized marijuana broadly enough to cause the pot-price drops needed to make the analysis. That might not happen in Colorado or Washington, either, if the federal government squashes forthcoming retail marijuana sales in the states, Kleiman said.

But two studies out of the University of Colorado Denver provide hints about what might happen.

In a paper soon to be published in the Journal of Law and Economics, professor Daniel Rees finds that traffic fatalities drop when states pass medical-marijuana laws. Rees also reports a drop in alcohol consumed by people ages 20-29 in medical-marijuana states.

Professor Benjamin Crost finds a similar relationship in a paper that argues marijuana use decreases and alcohol use increases after young people hit the legal drinking age.

“We should expect that the higher availability of marijuana in Colorado will lead to a decrease in alcohol use among young people,” Crost wrote in an e-mail.

That’s certainly what supporters of marijuana legalization — who frequently used the campaign message that marijuana is safer than alcohol — say will happen.

Kilmer, though, cautions that the relationship might be different in for people of different ages, genders and backgrounds. And Kleiman points out that the relationship may change over time — as the price of or stigma surrounding marijuana evolves. That’s what makes the crucial question still so elusive.

“I don’t think there’s any logical basis for knowing,” Kleiman said. “It’s a big, complicated change.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold