Many of the same people who criticized Attorney General William Barr for showing leniency towards Roger Stone and Michael Flynn in their criminal matters are now calling for the Biden Department of Justice to prosecute President Trump. 

This raises two questions fundamental to our legal system: Who in the federal government decides whether to prosecute alleged wrong-doers? And what criteria frame this analysis? 

The answer to the first question is easy: Federal prosecutors. This awesome power to take another citizen’s liberty is wielded by those at the DOJ.  

A civilized society must have a criminal justice system, which depends upon prosecutors bringing cases. While this is an enormous responsibility to vest in individuals who are largely untethered from democratic processes, there is no real alternative. 

The key question, then, is the second one: What criteria frames – and restrains – the decision-making regarding whether to bring a case?  

The discomforting answer is that prosecutorial discretion is primarily a matter of subjective human judgment. And human judgment in this context is just as ridden with bias and fallibility as in any other. The principle bias impacting prosecutorial discretion is confirmation bias – the strong propensity in human thinking to interpret new facts as consistent with existing beliefs.  

Prosecutors, of course, are a heterogeneous group and this bias impacts every prosecutor differently. Most prosecutors are well-intentioned and loyal public servants. But, as humans, they often interpret new facts uncovered during investigations as consistent with their preconceived notions and prejudices. In the arena of criminal justice – where life and liberty are at stake – this bias can have dramatic human consequences, especially when the target of an investigation is a controversial public figure. 

Bias in prosecutorial decision-making is, moreover, fundamentally at odds with the first principle of criminal justice – that the law applies equally to all individuals. Our nation has a long and scurrilous history of enforcing the criminal law one way towards favored groups and another towards disfavored groups.  

The Jim Crow system had explicitly racist laws on the books. But a law, alone, does no harm. The real damage was done by the unwritten biases of the prosecutors who chose which defendants to pursue. 

Disparate results still exist today. Of the two-plus million American inmates a staggering disproportion are minorities. The biases and prejudices of the people who bring the cases impact this disparity.  

Prosecutorial discretion is, indeed, an immense power. Unlike other decisions in our democracy that require an election among the people or a majority vote of our representatives, deciding whether to prosecute comes down to individual human judgment, largely unobserved and unchecked. 

Different people looking at the same set of facts often come to radically different conclusions. It’s no different with lawyers. Whether someone is prosecuted or not thus often boils down to the luck of the draw – to which prosecutor happens to be on the case.  

Many of the people calling for Trump's prosecution have long been proponents of criminal justice reform and reining in unfettered prosecutorial discretion. Sure, they say, the legal system is flawed; so only "clear crimes" committed by Trump should be prosecuted.  

But what is a clear crime? In the House of Representatives, 100 percent of Democrats found Trump committed an impeachable offense (a high crime) and 100 percent of Republicans found that he did not. It’s not that simple, especially when the visceral prejudices of politics are involved. 

There will never be an algorithm that determines – yes or no – whether someone should be prosecuted. And, yet, we depend upon prosecutors bringing cases, including against politicians, to keep society functioning. 

So how, then, should a prosecutor (or anyone else) conclude whether another citizen's liberty should be jeopardized by a prosecution? With profound caution. And this requires a decent respect for the inherent biases in human judgment. Even – indeed, especially – when the potential defendant is a political opponent. 

William Cooper is an attorney who opines on U.S. law and politics and has written for The Wall Street Journal, Baltimore Sun, New York Daily News and USA Today, among others.