REUTERS/Tomas Bravo

Over the last few years, supporters and opponents of marijuana legalization have both cited the same statistic to back their cause: 60% of the profits made by violent Mexican drug lords come from marijuana. But now, it seems, both sides have been wrong.

Nearly 28,000 deaths have been associated with Mexico’s war on drugs, which shows no signs of slowing. To mitigate the slaughter, supporters of legalization have argued that taking marijuana out of the equation by making it legal would hit kingpins where it really hurts, assuming the 60% figure is accurate. This argument has recently gained new currency with Proposition 19, the campaign to legalize marijuana in California via the state initiative that will be voted on in November. (More on Time.com: Prop 19 Analysis: Will Marijuana Legalization Increase Use).

Opponents of legalization, alternatively, have used the same 60% statistic to argue that marijuana is not a harmless or humorous drug, but one that is associated with all the evils of hardcore drug trafficking.

Where does the 60% figure come from? It was released by the Office of National Drug Control Policy — the federal drug czar —in 2006, but its origins and exact derivation were not made public. With legalization advocates using it enthusiastically, however, the agency officially backed away [PDF] from it in September, claiming that the models on which it was based “are dated and may no longer apply.”

To most experts, the number never really made sense in the first place: for one, why would Mexican drug lords be reaping so much of their profits from marijuana, when the country is the source or key transit point for most of America’s cocaine, heroin and methamphetamine — drugs that are typically more profitable than marijuana? Nevertheless, the rhetorical use of the statistic kept it popular in the media. (More on Time.com: The Siege of Ciudad Juárez).

Now the Rand Corporation’s Drug Policy Research Center offers a more scientific and sensible estimate of the economics. According to its own analysis of drug smuggling across the U.S.–Mexico border, Rand researchers estimate that marijuana accounts for 15% to 26% of illegal drug revenue in Mexico, and say the 60% figure “should not be taken seriously.”

This takes some steam out of the legalization debate — on both sides. Given that California makes up an estimated 14% of the U.S. marijuana market overall, legalizing the drug in that state would have minimal impact on Mexican drug profits or violence, cutting drug export revenue by perhaps 2% to 4%.

“Legalizing marijuana in California would not appreciably influence the Mexican drug trafficking organizations and the related violence unless exports from California drive Mexican marijuana out of the market in other states. If that happens, then legalization could reduce some of the Mexican drug violence in the long run. But even then, legalization may not have much impact in the short run,” said Beau Kilmer, the study’s lead author, in a press release. (More on Time.com: California’s First Marijuana Factory?).

What that leaves out, of course, is the impact legalization in California may have on the rest of the U.S. If California legalizes marijuana, other states may follow suit and U.S. drug policy could change dramatically. Whichever direction policy goes, however, good numbers are a better basis for argument than exaggerations — no matter how much they might seem to support your case.