A committee of scientists reviewed research done over the past 35 years and found it was "not informative about whether capital punishment decreases, increases, or has no effect on homicide rates," said the report.
"Consequently, claims that research demonstrates that capital punishment decreases or increases the homicide rate by a specified amount or has no effect on the homicide rate should not influence policy judgments."
The report was issued by the NAS's National Research Council, which convened a Committee on Deterrence and the Death Penalty to look at available evidence on how the death penalty may affect murder rates.
A previous report by the NRC in 1978 found that "available studies provide no useful evidence on the deterrent effect of capital punishment."
In the decades since that report, "a considerable number" of studies have attempted to judge how well it works, or does not, and have reached "widely varying conclusions," the latest report said.
"Fundamental flaws in the research we reviewed make it of no use in answering the question of whether the death penalty affects homicide rates," said Daniel Nagin, professor of public policy and statistics at Carnegie Mellon University and chair of the committee that wrote the report.
"We recognize that this conclusion may be controversial to some, but no one is well-served by unsupportable claims about the effect of the death penalty, regardless of whether the claim is that the death penalty deters homicides, has no effect on homicide rates or actually increases homicides."
Until now, a key flaw in the research has been the failure to account for how punishments such as life in prison without the possibility of parole may affect homicide rates.
Also, a number of assumptions have hobbled previous studies, particularly by assuming that potential murderers actually consider the risk of execution and respond accordingly.
Instead, researchers going forward must perform more rigorous studies that assess how potential criminals view the death penalty and its likely effect on their actions, the report said.
Better methods for future research include collecting data that consider both capital and non-capital punishments for murder and doing studies on how potential murderers perceive a range of punishments in homicide cases, it said.
Just 15 percent of people who have received the death sentence since 1976 have been executed, "and a large fraction of death sentences are reversed," added the report.
The members did not examine the moral arguments for or against capital punishment, or the costs involved.
