US District Judge Jed S. Rakoff notes the near-ubiquity of plea bargaining:
In 2013, while 8 percent of all federal criminal charges were dismissed (either because of a mistake in fact or law or because the defendant had decided to cooperate), more than 97 percent of the remainder were resolved through plea bargains, and fewer than 3 percent went to trial. The plea bargains largely determined the sentences imposed. While corresponding statistics for the 50 states combined are not available, it is a rare state where plea bargains do not similarly account for the resolution of at least 95 percent of the felony cases that are not dismissed; and again, the plea bargains usually determine the sentences
The cause, he says, is a power imbalance between prosecutors and defense attorneys:
[W]hat really puts the prosecutor in the driver’s seat is the fact that he – because of mandatory minimums, sentencing guidelines (which, though no longer mandatory in the federal system, are still widely followed by most judges), and simply his ability to shape whatever charges are brought –can effectively dictate the sentence by how he publicly describes the offense. For example, the prosecutor can agree with the defense counsel in a federal narcotics case that, if there is a plea bargain, the defendant will only have to plead guilty to the personal sale of a few ounces of heroin, which carries no mandatory minimum and a guidelines range of less than two years; but if the defendant does not plead guilty, he will be charged with the drug conspiracy of which his sale was a small part, a conspiracy involving many kilograms of heroin, which could mean a ten-year mandatory minimum and a guidelines range of twenty years or more. Put another way, it is the prosecutor, not the judge, who effectively exercises the sentencing power, albeit cloaked as a charging decision.
The defense lawyer understands this fully, and so she recognizes that the best outcome for her client is likely to be an early plea bargain, while the prosecutor is still willing to accept a plea to a relatively low-level offense. Indeed, in 2012, the average sentence for federal narcotics defendants who entered into any kind of plea bargain was five years and four months, while the average sentence for defendants who went to trial was sixteen years.