Democrats in California have raised taxes on the rich again and again, and liberals claim it has no effect on taxpayer migration and does no harm to state tax revenue. A new study finds the opposite.

Stanford economists Joshua Rauh and Ryan Shyu analyzed how high earners responded to a 2012 referendum (Prop. 30) backed by Democrats that raised the top marginal rate on taxpayers with more than $1 million of income to 13.3% from 10.3%. The top rates on individuals earning more than $250,000 also rose between one and two percentage...