That the death penalty is administered in a fair and equitable fashion within the federal system is belied by the vastly divergent facts on the ground

The arbitrary nature of the death penalty as practiced in the US is laid bare in a new study that shows that just 2% of counties across the nation have generated most of the executions in the past 40 years.

A new report from the Death Penalty Information Center in Washington shows that of the 1,348 executions that have taken place in the US since the death penalty restarted in 1976, more than half originated in only 2% of counties. Under the US judicial system, the decision to launch a capital case lies with district attorneys at the county level.

Put another way, 15% of the counties of America have given rise to all the state executions to be carried out in the modern era. The argument that has been played out at the highest levels of US jurisprudence, that the death penalty is administered in a fair and equitable fashion within the federal system, is belied by the vastly divergent facts on the ground.

The finding of such unequal distribution – including the fact that all of the 3,125 inmates currently on death row in America came from just 20% of the counties – has potentially significant legal consequences. The US supreme court imposed a ban on the death penalty in 1972 on grounds that it was practised in an arbitrary and random fashion, and only allowed it to restart four years later once new guidelines had been issued to jurors.

But as Richard Dieter, director of the Death Penalty Information Center, points out, his study shows that the ultimate punishment is today anything but consistent in its application. "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 – it's nothing to do with the heinousness of the crime. There are wild disparities between counties."

Even within the state of Texas, which carried out 15 executions last year, the discrepancies are wide.

Just four of its 254 counties – Harris, Dallas, Tarrant and Bexar – account for almost half of the 492 executions Texas has carried out since 1976. Harris county alone, around the city of Houston, has carried out 115 executions, a record number that in no small part can be explained by the particular affinity for judicial killings of its then-district attorney in the 1990s.

Many of the counties leading the league tables are cities with larger population sizes, but that does not explain the distortion. The four counties of Texas produce nearly 50% of the state's executions while holding 34% of its overall population.

Other counties that produce a high volume of capital cases include St Louis in Missouri, Tulsa and Oklahoma counties in Oklahoma, and Maricopa in Arizona.

While nobody expects the US supreme court to reconsider the issue of arbitrariness any time soon, politically, the disparities are starting to have an impact because of their financial implications. The death penalty is not cheap: on average it costs about $3m to achieve a death sentence, and if you add up all the costs through to actual execution that figure can escalate to $30m.

Most of those costs are borne by taxpayers in the states that still apply the practice – last year only nine states executed prisoners. Within those states, 85% of counties have produced no death sentences at all in the past four decades, so their residents are carrying enormous costs with none of the perceived benefits of sending anyone to death row.

"People are starting to realise that not only is the death penalty highly expensive, it is also extremely narrowly applied," Dieter said.

That syndrome was evident in Maryland, which abolished the death penalty in May. One of the prime arguments used by abolitionists was that the massive costs of Maryland's death row were being carried by all of the state's taxpayers, when the overwhelming number of the inmates had come from just one county – Baltimore.