I may be splitting hairs, but causal versus subjunctive deterrence aren't mutually exclusive as described here. In fact causal deterrence is always subjunctive and subjunctive deterrence is always causal. The difference is on what you're focusing: the historical learning experiences versus the lessons learned from those experiences. When you talk about causal deterrence, you're talking about the learning experiences themselves - but they are learning experiences only because of the lessons learned. Meanwhile, when you talk about subjunctive deterrence, you are talking about the lessons learned - but the lessons always require a learning experience. Let's apply it to these examples:
An evil mastermind would immediately look up the source code, realize this, and put their criminal plans on hold until they've found a way to circumvent Robocop. They would be subjunctively deterred.
The evil mastermind knows how to interpret source code - but he has to have at some point learned how to do this. If he's like the typical skilled programmer, he has a long history of having run code and seeing what happens. Even if you think, "in principle he could learn language simply by reading the language specification", he needs to have learned how to read and understand language, which again involves lots and lots of experience.
A regular hoodlum wouldn't realize that anything was different until they started seeing other criminals riddled with bullets. They would be causally deterred from committing crimes when they saw the punishment actually being meted out.
In order for the regular hoodlum to learn from the experiences of others as he does here, he must draw an inference about the future, such as, "if I were to commit a crime then Robocop would shoot me" - which belief about the future behavior of Robocop conditional on his own behavior, if it deters him from commiting crime, is subjunctive deterrence.
I still think that the two types of deterrence ought to be distinguished, as they differ on the key point of whether a punishment has to actually happen in order for deterrence to take place.
Recall the hypothetical raised here on LW wherein the US, immediately after WW2, declares that any other country found to be pursuing nuclear weapons will be immediately nuked. Ignoring the many moral and practical difficulties of that policy, it at least is an example where leaders would strongly prefer subjunctive deterrence to suffice.
Why do those words go together?
Society - and for once, I'm using this term universally - teaches that, if you committed a crime, you should be punished.
But in some societies, we have an insanity defense. If you had a brain condition so that you had no - here it's a little vague - consciousness, or moral sense, or free will, or, well, something - then it would be cruel to punish you for your crime. Instead of going to prison, you should be placed somewhere where you can't hurt anybody, where professional physicians and counselors can study your case and try to reform you so that you can rejoin society.
Wait - so that isn't what prison is for?
No. Prison is to punish people. Is it any wonder that prison doesn't reform people, when we don't want it to reform them? Most people would be upset if prisoners could go in on Friday, and emerge, rehabilitated, on Sunday. When people say, "It would be cruel to punish people who aren't responsible for their actions", they are implicitly saying, "Prison is necessarily cruel; and that's good, because we should be cruel to criminals who are responsible for their actions."
But the more we learn about psychology and neuroscience, the further responsibility recedes into the distance.
Outcome-based justice argues that we should give up playing the blame game, because neuroscience keeps finding more and more proofs that things are "not our fault". Instead, we should write laws that deter crime.
You might think this is what we already try to do. But it isn't! Witness this confused article from the Brookings Institute, Cognitive Neuroscience and the Future of Punishment by O. Carter Snead. Snead objects to outcome-based justice. He summarized all of the arguments for it, yet managed to completely miss their point, concluding where he started from, saying that outcome-based justice is obviously bad because it could lead to being cruel to people who didn't deserve it. (Instead of only being cruel to the people who do deserve it, which is obviously what we want to do.)
Snead understands that outcome-based justice deters crime:
You might expect that Snead goes on to explain why these laws are bad things. But he doesn't! He assumes we can all see that these are obviously bad things.
The Wrath of Kahneman describes a study which asked whether people punish others in order to deter crime, and concluded, No. People are doing something else.
One theory is that people are trying to be fair. Everyone should get the same chances; everyone should get the same punishment for the same crime. John Rawls argues this explicitly in Justice as Fairness: Justice should not be utilitarian, but should instead be fair.
I believe Rawls' view is also the popular view of what "justice" means. And, I will argue in a later post, it is part of a pattern showing a deep divide between two different ways of using the word "ethics".
ADDED: Constant made the point that, while one part of outcome-based justice is preventing future harm from the criminal on the dock, another part is deterring harm by other criminals. This latter part does not benefit from punishing criminals who cannot be deterred. Thus, to optimally punish both criminals who can and cannot be deterred, the law requires a concept of moral culpability, and should punish criminals who can't be deterred more lightly. This is a better origin story for the linking together of morality and free will than the just-so story I'd come up with, so I plan on stealing it for my next post. (SilasBarta may have been trying to make the same point, but I found his comments impenetrable.)
(This post is laying groundwork for two other posts that will go in different directions, neither of which concerns justice.)