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. 

Defense attorneys are arguing that Tsarnaev did not act independently but was under the sway of his older brother Tamerlan, who died during the police pursuit. This is what they must argue to avoid execution, and I’m firmly on their side. But their argument could go further: Tsarnaev was acting under the influence of his genes and his environment, of which Tamerlan was a part, and he had no choice other than to bomb the Boston Marathon.

Of course, such an argument is highly unlikely to sway a jury. But to see how the notion of pure libertarian free will is used by prosecutors asking for execution, here’s an excerpt from Wednesday’s New York Times article on the Tsarnaev case. I’ve highlighted the parts that suggest Tsarnaev had a real choice about what he did: 

Millions of people come from dysfunctional families, Ms. Pellegrini said, but they do not blow past normal boundary lines to become murderers, as Mr. Tsarnaev did. “The lines he was willing to cross make him fundamentally different,” she told the jury.

And he should not be able to shirk responsibility for heinous crimes that he committed by blaming someone else, she said. She quoted Shakespeare (“The fault, dear Brutus, is not in our stars, but in ourselves”) to convey that people choose the lives they lead.

“His destiny was determined by his actions, and he was destined and determined to be America’s worst nightmare,” she said. He “twisted the marathon into something cruel and ugly for his own purposes.”

There are of course good reasons to punish people like Tsarnaev, even if one is a determinist. Punishment keeps someone who is liable to do further damage away from society (sequestration); it serves as a deterrent to others (even though execution isn’t a deterrent, being caught and imprisoned is, as we can see from what happened during the famous Montreal Police strike of 1969); and in some cases (but probably not Tsarnaev’s) it’s possible to rehabilitate offenders when they’re confined, so that they pose little danger to society when they’re released. The effects of each of these rationales can in principle be judged by science, though the “experiments” will be hard and expensive. Still, a good society must surely try.

But vengeance or retribution are not valid reasons for execution, for both of those involve the notion of moral culpability—the idea that the guilty party had a choice and made the wrong one. Pandering to a mob or posse mentality demanding “an eye for an eye” tacitly accepts an emotion no longer tenable in an enlightened society. Yes, some may feel the need for vengeance, but it’s wrong to act on it. 

The fault, dear Brutus, is indeed in our stars—or rather in our genes and our circumstances. Tsarnaev was simply unlucky in what his parents and his life vouchsafed him, and he wound up an odious and murderous person. For that he should be put away for life, as the possibility of rehabilitation seems slim. But let’s not pretend that he could have done anything other than place those bombs.

A version of this post first appeared on WhyEvolutionIsTrue.