Would these states have lower murder rates whether any state at all had the death penalty or not? This is like comparing the gun related deaths rate of the UK vs the US and saying it is because of the gun laws passed in the 1990's in the UK....... while not finding out that even in 1910, well before any kind of gun laws anywhere the UK had a far lower rate than the US.
While there are a few exceptions (HI, AK, WV), most of the non-death penalty states are in the northeast or upper midwest. There are other factors that can cause a lower murder rate than just the death penalty.
Why do they start the comparison in 1990 when the death penalty was re-instated in 1976?
1990 is when the non-death penalty states peaked in murders, thus they were the closest they would ever be to the death penalty states, thus making any later changes in rates more drastic. A better comparison would have been to compare 1975or 1976 to present.
I compared the rates in 5 year intervals (1960, 1965, 1970, etc.) up to 2005 and then 2009. I didn't add up the populations and figure out a true per capita average, I averaged the states as if they were all equal (a state with 5.0 and a state with 10.0 would average to 7.5). What may work for high population states of New York and California may not work for lower population states.
There is faults with this method as well (Wyoming is treated the same as California), but any method has it's flaws. If a high population state such as New York sees a huge decline in murder rates by not having the death penalty, while a low population state has a huge increase then it makes sense that NY would not want the death penalty and the low population state would want it, but if you are just looking at total population numbers, the result is skewed by the results of high population states.
Percent change in Murder rate
...................................States with.............
.......................No Death...............With Death
Years...................Penalty...............Penalty
1965-2009...........+12.53%..............-11.22%
1970-2009...........-28.37.................-38.71
1975-2009...........-41.70.................-47.91
1980-2009...........-38.29.................-47.29
1985-2009...........-25.03.................-30.96
1990-2009...........-57.49.................-38.26
1995-2009...........-51.03.................-35.90
2000-2009...........-27.61.................-05.51
In 1975, nearly half of the death penalty states (17/36) had a murder rate over 10, but in the non death penalty states there was only 3/13 (note that in 1975 there was no death penalty in any state).
Number of states who saw a decrease of murder rate from 1975-2009
Percent...No Death ............With Death
Drop.......Penalty...............Penalty
70%+......2.......................3
60-70%...0.......................2
50-60%...2.......................12
40-50%...2.......................7
30-40%...2.......................7
20-30%...3.......................4
10-20%...0.......................0
0-10%....1........................1 (LA -6%)
Worse.....1 (ND +87%*).......0
All my calculations excludes Kansas and New York since they changed death penalty status in mid '90's.
New York's murder rate dropped after re-starting the death penalty, but it already was on the decline anyways due to other reasons. NY peaked in 1990 but the rate was already cut in half by the time they restarted the death penalty, and you can't say that not having the death penalty is responsible for that decline unless you want to say that not having it was responsible for it peaking in the first place!).
*If you think ND +87% for not having the death penalty is bad then keep in mind that if I had done 1970-2009 then it was +200%, and if I had done 1980-2009 then South Dakota (with death penalty) was +271%. (A change from 0.7 in 1980 to 2.6 in 2009), but in 1975 (with no death penalty) SD had a rate of 3.7.
Any particular state you could pick certain dates to show an increase or a decrease to support whichever political stance you want to support. Some states would go from a rate of 5.0 to 10.0 in one year, and then back down to 5.0 the next year and the death penalty was certainly not a factor. Whether you compare 1975-2009 or 1980-2009, all states saw a decline of murder rates (except the Dakotas, with SD having the Death Penalty and ND not having it)
Most death penalty states peaked in 1975-1980, so perhaps they saw the need for having the death penalty. The non-death penalty states, if they didn't peak in 1975 (6 states), they peaked in 1990 (DC, WI, RI and NY). Only 2 death penalty states peaked in 1990, (CT and LA), since many saw their peak already in 1975-1980.
Edit: Source comes from this site:
http://www.disastercenter.com/crime/nycrime.htm
Just change 'ny' in the link to the abbreviations for the state you want (KN is for kansas rather than KS), and sometimes the e in crime needs to change to n. NY is missing 1960 data and why I started at 1965.
Edit 2: Conclusion
I can see (and perhaps agree with) the argument that the death penalty has no effect on murder rates, but I don't really see the evidence that the death penalty INCREASES murder rates.