For help understanding (and answering) these questions, I asked John H. Blume, a professor at Cornell Law School who directs the school's respected Death Penalty Project. In essence, I asked who was right and why. Here's Blume's response:
Every credible study has found a statistically significant race of victim effect (i.e., more death sentences are imposed in white victim cases), and the effect is most pronounced in black defendant/white victim cases. I have written about this, as have many others, but the effect is real, and it does not disappear when other factors are variables are taken into account.
[Lott] is right about whites being sentenced to death at a higher rate in the aggregate than African-Americans but this is a result of the same phenomena. Whites overwhelmingly kill whites, and black overwhelmingly kill blacks. So, since the overall death sentencing rate is higher in white victim cases, more death sentences in the aggregate will be imposed on white defendants.
But, the most death sentences (again the effect is statistically significant) is found in white victim/black defendant cases, and this effect is most pronounced in the Southern states that were part of the Old Confederacy (but is present in all states that retain the death penalty).
Blume also debunked Lott's claim about the deterrent effect:
As for the claim that each execution saves 15-18 potential murder victims, it is, as my recently deceased colleague and the grandfather of empirical legal studies Ted Eisenberg said after reading it, “preposterous.” There have been numerous studies over decades attempting to establish whether there is (or is not) any deterrent effect from sentencing persons to death and executing persons sentenced to death.
No reliable study by credible researchers has ever found any deterrent effect. And the idea that you could pinpoint a relatively precise number of persons whose lives were saved by an individual execution is sophistry of the highest order.
Both Balko and Lott cited the work of Samuel Gross, a professor at the University of Michigan Law School, who just helped publish an important study estimating "that if all death-sentenced defendants remained under sentence of death indefinitely at least 4.1 percent would be exonerated. We conclude that this is a conservative estimate of the proportion of false conviction among death sentences in the United States."
I asked Gross essentially the same question I asked Blume. On deterrence, he told me, we have Lott, who argues that "innocent people’s lives are saved thanks to the death penalty," and then we have an April 2012 report from the National Academy of Science's National Research Council (my emphasis):
Research to date on the effect of capital punishment on homicide rates is not useful in determining whether the death penalty increases, decreases, or has no effect on these rates, says a new report from the National Research Council. The committee that wrote the report evaluated studies conducted since a four-year moratorium on the death penalty was lifted in 1976, and it found that the studies do not provide evidence for or against the proposition that the death penalty affects homicide rates.
These studies should not be used to inform judgments about the effect of the death penalty on homicide, and should not serve as a basis for policy decisions about capital punishment, the committee said.
The lack of evidence about the deterrent effect of capital punishment—whether it is positive, negative, or zero—should not be construed as favoring one argument over another, the report stresses. "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 S. Nagin, Teresa and H. John Heinz III University Professor of Public Policy and Statistics at Carnegie Mellon University Pittsburgh, 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."
Gross also disputed Lott's characterization of his new National Academies of Science paper on exonerations and the death penalty. "I wonder if he bothered to read it," Gross said in an email Tuesday. He continued:
Lott says: "Many times guilty people go free because evidence that was used at trial is later determined during an appeal to have been improperly obtained. The paper co-authored by Gross defined 'exoneration' as occurring when someone was removed from death row by a 'legal action by courts or executive officials.'"
This is flawed, Gross alleges, for the following reasons:
1) Many—most—death sentenced prisoners have their convictions, or more often their sentences, reversed for all sorts of technical reasons. But few are cleared and freed.
(2) We do NOT define an exoneration as a "removed from death row." That's just false. We say specifically and repeatedly that most death sentences inmates are removed from death row but not exonerated—in fact our methodology depends on that difference.
(3) We do define an exoneration for this study as a case in which someone who was sentenced to death has been completely cleared of the offense for which s/he was sentenced to death and freed (except for a few who had died). We claim that in those cases (as opposed to "was removed from death row") the defendants are overwhelmingly factually innocent. But we also have a section on sensitivity analyses in which we address the consequences of mis-classifications, and we conclude that we have produced a conservative estimate.
It is clear that the debate over the death penalty in America, which preceded the Declaration of Independence and the drafting of the Constitution, will continue long after Balko and Lott and Blume and Gross (and I) are gone. But no one now or later benefits from a debate based in whole or part on what the NAS calls "unsupportable claims about the death penalty."
I'll leave the fighting over stats to the academics. I know what I cover every day on this beat: The death penalty as currently implemented in America is both unconstitutional and immoral, because too many capital trials are unfair, too few appellate reviews are meaningful, and too few judges, prosecutors, witnesses, and politicians care enough to change that.