Cost of death sentences nationwide since 1973 has been estimated at $25B 2009 Virginia Department of Corrections

More than half of all executions carried out in the U.S. stem from cases in just two percent of counties, yet taxpayers nationwide have shouldered the estimated $25 billion cost of death row sentences since 1973, a new study found.

Death sentences are at their lowest level in four decades – 85 percent of all counties have not had a single person executed in more than 45 years according to the study – but criminal offenders in the states of California, Texas, Oklahoma and Florida have a disproportionate chance of ending up on death row, with a small number of counties responsible for a majority of death penalty sentences across the country.

Research also revealed that just 62 of the 3,143 counties in the U.S. were responsible for 52% of all death row inmates executed since 1976.

The authors of the study say the results dispel the view that the death penalty is widely practiced across the U.S. and that the use of execution is well supported.

"People here believe in the death penalty but not in a death penalty that’s unfair," Richard Dieter, executive director of the Death Penalty Information Center and the study's author, told Al Jazeera. "And that’s the one that exists in practice."

The South accounts for more than 80 percent of executions, with Texas leading the fray at 38 percent. But even within these states, great internal disparities exist.

"It appears in clusters, in certain cities, in certain counties," he said. Just four counties – Harris, Dallas, Tarrant and Bexar – account for almost half of the state's executions, but only represent 34 percent of the population.

In 2009, just three counties in California – Los Angeles, Orange and Riverside – accounted for more than 80 percent of the state's executions, but represented less than half of the population.

While Dieter pointed at a strong support for capital punishment on religious grounds, his research also revealed another, more hidden reason for the regional discrepancy.

Often, the decision to seek the death penalty rests with a single prosecutor, who is unlikely to be held accountable in the case that his judgment appears misguided. Subjective interpretations on whether the crime was "heinous, cruel or atrocious," an aggravating circumstance necessary to justify pursuing the death penalty, can lead to various outcomes.

"The discretion to seek the death penalty is totally within the prosecutor's realm and that’s where abuse may come in," he said. "Obviously as an individual, he might have biases," Dieter added.

Factors such as race and geography appear to be more significant in a prosecutor's decision than the actual severity of the crime, the report found.

A 2012 study into the influence that race may have had in the capital murder case of Duane Buck, who was convicted of shooting three people and killing two in 1995, found that juries in Harris County, Texas, were more than twice as likely to seek the death penalty for black defendants as white defendants in similar cases.

"Yet it's the same law within a state that should apply to all," Dieter said.

Prosecutors, who are typically elected, are also not immune to electoral pressures and might seek to boost their approval rates by cracking down on high-profile murder cases. "Some studies have shown an increase in death penalties around election time," he said.

Sentencing people to death by execution is three times more expensive than sentencing them to die in prison, according to the American Civil Liberties Union. But counties that use the death penalty bear only a small fraction of the costs. The authors of the report estimated that the cost of death row sentences to U.S taxpayers since 1973 has been $25 billion. It is a “staggering sum for the 85% of U.S. counties that have not had a single case resulting in an execution,” the report notes.

"A few people are driving costs and transferring them to a large majority," Dieter said.