Capital punishment fell by more than one-third around the world last year – and the United States dropped out of the top 5 countries for the first time in a decade.

That was the finding of Amnesty International's annual report on the world's death sentences and executions in 2016. Compared with 2015, the human rights group found that capital punishment had decreased by 37 percent. In the US, the number of people executed – 20 – fell to levels not seen since 1991. The drop marks the first time since 2006 that the US has not been in the top five.

Over the past few decades, the international view of capital punishment has shifted considerably, with more than two thirds of all nations no longer supporting the death penalty, either legally or in practice. But some countries, particularly China, Iran, and the US, maintain the practice as a legal mode of punishment.

The debate surrounding the death penalty in the US is a contentious one. Last fall, for instance, voters in California, Nebraska, and Oklahoma decided to keep the penalty legal in their respective states, despite overall support for the death penalty reaching lows not seen since it was reinstated in the 1970s. But that ambivalence might not remain for much longer, according to Robert Owen, a law professor at Northwestern University in Evanston, Ill.

"The continuing decline in executions in the United States results from a combination of factors," he tells the Monitor via email. "For example, innocent people continue to be identified and released from death row, which understandably undermines support for capital punishment. Also, historic concerns about racial injustice in the administration of the death penalty are once again receiving attention with the rise of activist movements such as Black Lives Matter."

Professor Owen says that another, less ideological factor in the decline of the death penalty is an ongoing shortage of lethal injection drugs in the US, which in some cases led to drug cocktails that created botched executions that have "contributed to the growing skepticism" toward the practice.

In order to contend with these shortages, however, many states, especially in the South, have legalized older methods, such as gas chambers, electric chairs, or firing squads. Arkansas plans to execute an unprecedented seven prisoners over 11 days starting next week in order to beat the upcoming expiration date of the state's remaining lethal injection drugs.

Most states occupy a middle position on the death penalty: Often, they will sentence criminals to death, but not actually carry out the execution.

"Recent polling shows the lowest level of support for the death penalty since the 1970s," says Griffin Hardy, communications coordinator for the Ministry Against the Death Penalty, a Catholic anti-capital punishment group, in a phone interview. "Referenda on the death penalty in California, Nebraska, and Oklahoma may indicate continued public support for keeping the death penalty on the books, but it is worth noting that each of these states faces significant obstacles in resuming executions. The death penalty is an empty promise to victims' families in the majority of states that legally retain the option to execute prisoners."

Hardy says that the death penalty in the US is on the way out – not a question of "if," but "when."

But countries like China, the No. 1 executor of criminals, may not let go of practice as quickly.

"China retains the death penalty as a legal punishment for a wide range of crimes, including some nonviolent offenses [and political suppression]," Hardy says. "It is difficult to gauge trends in China's use of the death penalty due to the scarcity of statistics, but it seems that the number of executions is down from the ten-thousands to the thousands."

By comparison, Iran, which holds the No. 2 spot, executed 567 people last year. In third place is Saudi Arabia, with 154 executions, with Iraq trailing in fourth place, with 88.

"A record high number of executions were carried out worldwide in 2015, so the decrease in 2016 must be viewed in that context," Mr. Hardy points out.

While the US is not the only country in the Americas to assign the death penalty to criminals in 2016, it is the only country in the region to have actually carried out the death penalty in the past eight years, according to the Amnesty report. In the US, 2,832 people were living on death row as of the end of 2016.

"Use of the death penalty in the USA is at its lowest since the early 1990s," said Salil Shetty, secretary general of Amnesty International, in a statement. "But we have to fight to keep it that way."