There are many reasons to oppose the death penalty, some more convincing than others. It involves the state in an act of killing (an argument that, by itself, I don’t find all that convincing); it does not act as a deterrent to others (more convincing); because of the appeals process, it costs more than a sentence of life in prison without parole; it’s disproportionately given to blacks in the U.S., and so is racist; sometimes the condemned suffer horrible, tortured deaths; sometimes innocent people are executed, and there’s no way to right that wrong.

But one argument is rarely used. Determinists like me argue that criminals don't have any choice in what they do, and therefore excusing people from death because they were “cognitively impaired,” “didn’t know right from wrong,” or had other extenuating circumstances is no more valid than excusing people “because they have a brain that obeys the laws of physics.” In other words, if you exculpate one person from execution on any grounds of cognitive impairment, then you must exculpate all of them, for nobody can ever make a free choice between killing or refraining from it. In some sense all criminals are cognitively impaired, for, like the rest of us, their actions were determined completely by their genes and environment; at no time, were the tape of life rewound, could they have behaved differently.

This, of course, does not mean that such people should be let off scot-free—far from it. But there is no good reason to execute people for retribution, or on the grounds that they made a free choice, with sound mind, to kill someone else. That would imply that we have real libertarian choices. But if you have no such choices, while you might be responsible for a crime, you are not morally responsible. Moral responsibility implies the ability to have done otherwise.

Yet “moral responsibility,” and the implication that killers could have chosen to do otherwise, is one of the most important reasons given for putting people to death in the U.S. At this moment, a jury in Massachusetts is weighing imposing a federal death penalty on 21-year-old Boston Marathon bomber Dzhokhar Tsarnaev after he was convicted on all 30 criminal counts. The federal death penalty was reinstated in 1988 and since then three people have been executed (including Oklahoma City bomber Timothy McVeigh), while 44 have been given the penalty and are languishing on death row.

But, as the Boston Globe notes, Tsarnaev’s circumstances are special because the bombing is seen as a terrorist act—a public one and a gory one. He may well be sentenced to death and executed. Attorney General Eric Holder made the decision to request the death penalty, and he’s supported by several maimed victims and relatives of those who died. If the jury rules unanimously for death, it’s curtains for Tsarnaev; otherwise he goes to jail for life, without the possibility of parole.