California lost as many as 138 high-income individuals after the Proposition 30 income tax increase backed by Democratic Gov. Jerry Brown was approved by voters in 2012.

Charles Varner, associate director of the Stanford Center on Poverty and Inequality, released the information on Friday, Forbes reported.

The new report updates information published six years ago that examined migration to and from California after a tax increase in 2004.

"One reason we wanted to update our previous paper is that this tax change in 2012 is the largest state tax change that we have seen in the U.S. for the last three decades," Varner told Forbes.

Known as Prop. 30, the law raised California's top income tax rate by 8 percent, increasing it a percentage point to 13.3 percent, making it the nation's highest state income tax rate.

It also hiked the tax rate on income of $300,000 to $500,000 by two percentage points, while increasing the rate on income of more than $500,000 by three percentage points.

In 2016, California voters extended the Prop. 30 income tax increases to 2030. They were scheduled to expire next year, Forbes reports.

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