By GEORGE SOROS

Our marijuana laws are clearly doing more harm than good. The criminalization of marijuana did not prevent marijuana from becoming the most widely used illegal substance in the United States and many other countries. But it did result in extensive costs and negative consequences.

Law enforcement agencies today spend many billions of taxpayer dollars annually trying to enforce this unenforceable prohibition. The roughly 750,000 arrests they make each year for possession of small amounts of marijuana represent more than 40% of all drug arrests.

Regulating and taxing marijuana would simultaneously save taxpayers billions of dollars in enforcement and incarceration costs, while providing many billions of dollars in revenue annually. It also would reduce the crime, violence and corruption associated with drug markets, and the violations of civil liberties and human rights that occur when large numbers of otherwise law-abiding citizens are subject to arrest. Police could focus on serious crime instead.

The racial inequities that are part and parcel of marijuana enforcement policies cannot be ignored. African-Americans are no more likely than other Americans to use marijuana but they are three, five or even 10 times more likely—depending on the city—to be arrested for possessing marijuana. I agree with Alice Huffman, president of the California NAACP, when she says that being caught up in the criminal justice system does more harm to young people than marijuana itself. Giving millions of young Americans a permanent drug arrest record that may follow them for life serves no one’s interests.

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Racial prejudice also helps explain the origins of marijuana prohibition. When California and other U.S. states first decided (between 1915 and 1933) to criminalize marijuana, the principal motivations were not grounded in science or public health but rather in prejudice and discrimination against immigrants from Mexico who reputedly smoked the “killer weed.”

Who most benefits from keeping marijuana illegal? The greatest beneficiaries are the major criminal organizations in Mexico and elsewhere that earn billions of dollars annually from this illicit trade—and who would rapidly lose their competitive advantage if marijuana were a legal commodity. Some claim that they would only move into other illicit enterprises, but they are more likely to be weakened by being deprived of the easy profits they can earn with marijuana.

This was just one reason the Latin American Commission on Drugs and Democracy—chaired by three distinguished former presidents, Fernando Henrique Cardoso of Brazil, César Gaviria of Colombia and Ernesto Zedillo of Mexico—included marijuana decriminalization among their recommendations for reforming drug policies in the Americas.

Like many parents and grandparents, I am worried about young people getting into trouble with marijuana and other drugs. The best solution, however, is honest and effective drug education. One survey after another indicates that teenagers have better access than most adults to marijuana—and often other drugs as well—and find it easier to buy marijuana than alcohol. Legalizing marijuana may make it easier for adults to buy marijuana, but it can hardly make it any more accessible to young people. I’d much rather invest in effective education than ineffective arrest and incarceration.

California’s Proposition 19, which would legalize the recreational use and small-scale cultivation of marijuana, wouldn’t solve all the problems connected with the drug. But it would represent a major step forward, and its deficiencies can be corrected on the basis of experience. Just as the process of repealing national alcohol prohibition began with individual states repealing their own prohibition laws, so individual states must now take the initiative with respect to repealing marijuana prohibition laws. And just as California provided national leadership in 1996 by becoming the first state to legalize the medical use of marijuana, so it has an opportunity once again to lead the nation.

In many respects, of course, Proposition 19 already is a winner no matter what happens on Election Day. The mere fact of its being on the ballot has elevated and legitimized public discourse about marijuana and marijuana policy in ways I could not have imagined a year ago.

These are the reasons I have decided to support Proposition 19 and invite others to do so.

Mr. Soros is chairman of Soros Fund Management and founder of the Open Society Foundations.

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