Just 15 percent of counties account for all modern executions. The decision to bring a capital case against a defendant originates with District Attorneys at the county level.
Four of the 254 counties in Texas account for nearly half the state's 492 executions since 1976 -- Harris, Dallas, Tarrant and Bexar. Harris County, around Houston, carried out a record 115 executions on its own.
Although many of the leading counties contain cities with large populations, it doesn't entirely explain the disparity.
Other counties that prosecute a high volume of capital cases include St. Louis in Missouri, Maricopa in Arizona, and Tulsa and Oklahoma counties in Oklahoma.
Last year, only nine states carried out executions. Even within those states, 85 percent of counties delivered no death sentences at all in four decades.
Richard Dieter, director of the Death Penalty Information Center, says the study shows that capital punishment is inconsistently and arbitrarily applied.
“It is becoming clear that it comes down simply to which side of the county line you were standing in when you committed a murder that can put you on death row," Dieter said. "It’s nothing to do with the heinousness of the crime. There are wild disparities between counties.”
Further, the cost of capital punishment is borne by taxpayers in the states issuing the death sentences. Achieving a death sentence costs about $3 million on average. Add up all the costs of housing prisoners through to actual execution and that figure can rise to $30 million.
“People are starting to realise that not only is the death penalty highly expensive, it is also extremely narrowly applied,” Dieter said.