When California voters check off their ballots next month, they’ll have the opportunity to make their state the eighteenth in the nation to abolish the death penalty. So far, proponents of the abolition ballot measure, Proposition 34, have stressed the $130 million the state could save each year by changing the maximum sentence to life without parole as well as the $100 million that would be set aside for law enforcement to work on unsolved crimes.

However, there’s a more direct reason for abolition: More defendants have been wrongfully convicted and exonerated in California than any other state in the nation -- indicating significant risk of putting an innocent person to death.

Until recently, we didn’t know that this was the case. At the National Registry of Exonerations, where I work as an investigator and writer, Illinois originally held the dubious distinction of having the most exonerations, with 111 since 1989. In September, however, California overtook it. The California count now stands at 120 exonerations since 1989.

Meanwhile, today the University of California at Berkeley School of Law and Hollway Advisory Services, a criminal justice research firm, announced the launch of the California Wrongful Convictions Project, an effort dedicated to identifying wrongful convictions in California and assessing their economic impact.

“Wrongfully convicted Californians have spent more than 1,300 years in prison, costing taxpayers more than $129 million for unnecessary incarceration and compensation,” the Project announced in a press release.

With 726 inmates sentenced to die, California has the largest death row in America. The state has executed just thirteen death row inmates since capital punishment was reinstated in 1977, and none since 2006. The path from conviction to lethal injection for those who were executed has taken as long as twenty-five years.

In May, the National Registry released a report describing the first 873 exonerations it identified – including seventy-nine state exonerations and one federal exoneration in California. The Report emphasized that the 873 were only a beginning—that the true number of exonerations still is unknown because there is no formal system for recording such cases as they occur.

Since then, the number of exonerations on the National Registry has grown to 996 and will soon top 1,000, according to Samuel Gross, Law Professor at the University of Michigan and editor of the Registry.

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As the result of a focused investigation, the number of known exonerations in California has grown more rapidly than in any other state, from eighty to 120 in just a few months. “California is a very large state,” said Gross. “We knew there were a lot of false convictions and exonerations that we hadn’t heard about. We’ve found some of them, but there are many others out there.”

The Berkeley-Hollway report is preliminary. A full study is due in 2013. The project includes some cases that do not qualify under Registry’s criteria, such as cases that were dismissed prior to conviction or acquittals for insufficient evidence. So far they have documented 213 Californians who were imprisoned on felony charges that were ultimately dismissed or resulted in acquittals. Forty of them were sentenced to death, life or life without parole for convictions that were later overturned or dismissed.

“This data shows that both inadvertent mistakes and deliberate misconduct are more common in our justice system than most people think,” said John Hollway, founder of Hollway Advisory Services. “We are seeing prosecutors and defense attorneys work hand in hand to reduce these errors. A systematic review of the mistakes will allow us to measure the frequency and costs of justice system flaws, especially in this era of prison overcrowding and cost overruns.”

Will it make a difference in California?

Proposition 34’s proponents certainly think it will help them make their case. “Voters in California now have another powerful set of facts to support a Yes on 34 vote—the 120 innocent people wrongly convicted in California just since 1989,” said Natasha Minsker, manager of the Yes on 34 campaign. “This research shows that a shocking number of innocent people have been wrongly convicted in California and that we continue to make the kind of mistakes that send innocent people to death row.”

The polling showing public support for the death penalty may not be as solid as it first seems. A Los Angeles Times poll showed that when voters were asked about banning “the death penalty,” 51 percent opposed the idea and 34 percent supported it. But when voters were read a summary of the proposition -- including the plan to set aside $100 million for police investigations and requires prisoners to work and pay restitution to victims’ families -- and asked how they would vote, the result was a statistical dead heat, with 12 percent of the voters undecided on the measure.

On Monday, the Yes on 34 campaign released several television and radio advertisements. Perhaps the most powerful is that in which former Los Angeles County District Attorney Gil Garcetti declares, “California’s death penalty serves no useful purpose—and I should know—I prosecuted death penalty cases for three decades of my life.”

Echoing the findings of the Exoneration Registry, Garcetti adds, “After a lifetime in law enforcement, I have come to realize that life in prison without parole is the better choice because it means we will never make a fatal mistake.”

Maurice Possley, a Pulitzer Prize winning former investigative reporter for the Chicago Tribune, is an investigator and writer/researcher for the National Registry of Exonerations.