There's a nice paper by the philosopher David Lewis (http://andrewmbailey.com/dkl/Punishment_Chance.pdf is a copy of it; I don't know whether it's in any sense a legal copy) that offers (though it doesn't exactly endorse) the following justification for this:
1. Imagine a system where the punishment for murder is exactly the same as the punishment for attempted murder, but that punishment has an element of chance: you might get a longer or a shorter sentence.
This doesn't seem particularly unreasonable. Now we're going to make successive modifications to the system, which (Lewis claims) don't obviously make it worse.
2. Now suppose that we attempt to make the punishment for more wholehearted attempts at murder greater. It's hard to tell what was in the criminal's heart, so as a reasonable proxy we try to evaluate, somehow, how likely the murder attempt was to succceed, and we punish more-likely-to-succeed attempts more harshly.
3. Now suppose, more specifically, that the way in which we punish more-likely-to-succeed attempts more harshly is by increasing the probability of getting the longer sentence. In fact, we'll say that the probability of getting the longer sentence is to be the same as our estimate of how likely the attempt at murder was to succeed.
4. How to estimate that probability? One effective but obviously impractical way would be to run some sort of reenactment or simulation of the crime, and give the longer sentence if and only if the victim dies. Aha! But we can simplify and improve this by using the original crime as the reenactment, and give the longer sentence if and only if the actual victim died.
And now we've arrived at pretty much our present practice!
Doesn't seem compelling. After all, the root of the argument is about probabilities, but the argument then goes on to propose that we base our decisions on a single simulation of a result drawn from a weighted random distribution.
Specifically step 2 sounds like it’s skipping the important bit that people disagree with on the original problem. Again, compare hiring the cheapest hit man vs the most expensive.
1. Imagine a system where the punishment for murder is exactly the same as the punishment for attempted murder, but that punishment has an element of chance: you might get a longer or a shorter sentence.
This doesn't seem particularly unreasonable. Now we're going to make successive modifications to the system, which (Lewis claims) don't obviously make it worse.
2. Now suppose that we attempt to make the punishment for more wholehearted attempts at murder greater. It's hard to tell what was in the criminal's heart, so as a reasonable proxy we try to evaluate, somehow, how likely the murder attempt was to succceed, and we punish more-likely-to-succeed attempts more harshly.
3. Now suppose, more specifically, that the way in which we punish more-likely-to-succeed attempts more harshly is by increasing the probability of getting the longer sentence. In fact, we'll say that the probability of getting the longer sentence is to be the same as our estimate of how likely the attempt at murder was to succeed.
4. How to estimate that probability? One effective but obviously impractical way would be to run some sort of reenactment or simulation of the crime, and give the longer sentence if and only if the victim dies. Aha! But we can simplify and improve this by using the original crime as the reenactment, and give the longer sentence if and only if the actual victim died.
And now we've arrived at pretty much our present practice!