It is taken to be a platitude that one person can’t be morally responsible for the actions of someone else (see, e.g., Ted Sider’s book Four Dimensionalism for a recent reaffirmation of this claim). But there seems to be a fairly simple argument against this view, an argument drawing from Derek Parfit’s work on personal identity. Suppose that Adam has committed a brutal murder, and he then undergoes fission, that is, one half of his brain is put in one identical triplet’s body, and the other half is put in the other identical triplet’s body (call the two other triplets Brett and Carl). Let us then stipulate that each of the resulting persons is qualitatively identical to Adam. What’s happened to Adam? There are the usual four options: Adam survives as both, survives as Brett, survives as Carl, or doesn’t survive. There are clearly two persons in existence after the fission, though, so because one person doesn’t equal two, Adam can’t survive as both. And there’s no non-arbitrary reason for why he would’ve survived as Brett and not Carl, and vice versa. So the best description of the case is that Adam hasn’t survived the fission. But this isn’t even remotely as bad as ordinary death; indeed, what’s occurred to Adam is just as good as ordinary survival: both Brett and Carl will (quasi-)remember Adam’s commission of the crime, they’ll fulfill his intentions (at least insofar as their duplication won’t cause pragmatic conflicts in carrying them out), and they’ll persist in his beliefs, desires, goals, and general character.
But what about responsibility for his actions? If the loss of identity in this way is unimportant with regard to ordinary patterns of prudential concern, then why should it matter with regard to moral attributions like responsiiblity? What matters instead, it seems clear, are the relations of psychological connectedness that obtain between Brett/Carl and Adam. But insofar as these relations obtain to the highest possible degree, there seems no reason to deny that both Brett and Carl are responsible (or least quasi-responsible) for Adam’s actions, which is a straightforward denial of the so-called platitude: one person can be responsible for the actions of someone else.