Gross murder rates cannot be a valid and consistent method of measuring deterrence, for a variety of well know reasons (1).
A measurement of net changes in murders, meaning that there would be more or less net murders, based upon executions or the lack, thereof, is the proper calculation, whether or not gross murder rates, go up, down or stay the same, just as with deterrence, all crimes and all crime rates.
As many anti death penalty folks, including academics, wrongly, present some gross murder numbers as a foundation for arguing against deterrence, I present this:
HISTORY: MURDER RATES AND EXECUTION CYCLES
Again, this is not how deterrence is measured (1), however . . . .
1. The United States has had double digit executions, annually, from 1984 - 2011 (3).
Murders are, now, at a 43 year low (2),
Murder rates are, now, at a 48 year low. (2).
It's not surprising that death sentences are at a 37 year low (3).
2. Double digit annual executions stopped in the US in 1964 and resumed in 1984 (3).
During that period, murders increased by 100%
murders in 1964 9,360 (2)
murders in 1984 18,670 (2)
3. There was a moratorium on all executions in the US from 1967 to 1977 (3).
During that period, there was a 56% increase in murders (2).
murders is 1967 were 12,240 (2)
murders in 1977 were 19,120 (2)
Capital murders may have dropped by 80% or more in Texas since 1991 (2).
Murder in combination with rapes and/or robberies makes up, by far, the largest percentage of death penalty eligible murders.
Murders have dropped 58%, rapes 20% and robberies 41% (2).
A much lower occurrence of rape/murders and robbery/murders equals a much lower number of death sentences.
The per capita rape/murders and robbery/murders will have dropped even more dramatically, because Texas has seen a population explosion during this period.
Murders rates have dropped 71%, rapes 46% and robberies 61% (2).
The drop in capital murders is but the most obvious and pronounced explanation for a drop in death sentences.
Contributions to that drop also include:
1) Plea bargains to life without parole (LWOP) , a plea only possible with the death penalty;
2) Two classes of murderers have been excluded from the death penalty, that being those under 18 and those with mental retardation; and
3) Depending upon conditions within individual jurisdictions, with a downturn in the economy, the up front costs of the death penalty may have caused some jurisdictions to offer either plea bargains or lesser than death penalty option trials, to a degree more than in the past.
1) a) "Death Penalty, Deterrence & Murder Rates: Let's be clear"
NOTE: There are some deterrence studies which find a reduction in murders, soon after executions. Howevewr, I am, primarily, dealing with murders and murder rates for any given year.
2) United States Crime Data, from FBI UCR
The Disaster Center is a convenient and reliable source for crime data
murders rapes robberies
1991 2652 9266 47900
2011 1126 7439 28395
dif 1526 1827 19505
less 58% 20% 41%
murders rapes robberies
1991 24,700 106,590 687,730
2011 14,612 83,425 354,396
dif 10,088 23,165 333,334
less 41% 22% 48%
Source: Disaster Center, from FBI UCR
3) Capital Punishment, 2010 - Statistical Tables, Bureau of Justice Statistics, Tracy Snell, Dec 2011, see Figure 1, page 1 and Table 8, page 12,
OF COURSE THE DEATH PENALTY DETERS
LIFE: MUCH PREFERRED OVER EXECUTION:
99.7% of murderers tells us "Give me life, not execution"
See sections C and D within
The Death Penalty: Saving More Innocent Lives
"DEATH PENALTY DETERRENCE CLARIFIED"
Innocents More At Risk Without Death Penalty