The state constitutional amendment proposed by the Service Employees International Union and the Massachusetts Teachers Association to add a 4 percent surtax to all annual income above $1 million purports to use cost-of-living-based bracket adjustments as a safeguard that will ensure only millionaires will pay. But historic income growth trends suggest that bracket creep will cause many non-millionaires to be subject to the surtax over time, according to a new study published by Pioneer Institute.