Improper Arguments

Improper prosecutorial arguments are statements by the prosecutor—usually in opening statement or closing argument—that urge jurors to decide the case on an improper basis rather than on the law and the evidence. Common examples include commenting on a defendant’s silence, misstating the burden of proof, appealing to passion or fear, vouching for witnesses, invoking facts not in evidence, or encouraging jurors to punish the defendant for broader social problems rather than the charged conduct.

This is a trial-error issue. Unlike structural rules governing jury selection or verdict procedures, improper argument occurs during the presentation of the case to the jury and can therefore be evaluated for its effect on juror decision-making. Edwards’s work treats improper prosecutorial arguments as one of the recurring trial errors whose effects can, at least in some cases, be studied empirically rather than left entirely to intuition.

How can improper argument create unfairness?

Prosecutors do not merely summarize evidence. They help frame how jurors understand the case. That is why improper argument can matter even when the underlying evidence is unchanged.

An improper argument can make weak evidence seem stronger, shift attention away from reasonable doubt, encourage jurors to draw forbidden inferences, or turn deliberation into a referendum on fear, outrage, or sympathy. In close cases, those rhetorical moves may alter how jurors process the evidence and how confidently they vote.

The empirical literature recognizes that attorney arguments can affect outcomes. Judy Platania and Gary Moran’s study on prosecutorial misconduct in capital closing arguments is one example from a broader body of research showing that attorney arguments may affect conviction and punishment decisions. Attorney behavior often does not produce large direct effects but may still alter outcomes by providing jurors with a cognitive framework for interpreting the evidence.

Why improper argument does not always require a new trial

Not every improper statement warrants reversal.

Courts do not treat prosecutorial misconduct in argument as automatically fatal. The usual question is whether the improper remarks were serious enough, in context, to deprive the defendant of a fair trial or to create a level of prejudice that justifies relief. That inquiry is necessarily case specific. A comment that may be devastating in a thin case may have little practical effect in a case with overwhelming independent evidence. The same is true of repeated comments versus an isolated remark, or a statement tied closely to a contested issue versus a statement on the margins of the case.

Edwards’s Measuring Fairness article makes this point through side-by-side examples: one case in which improper prosecutorial comments produced a large estimated increase in the probability of conviction, and another in which the effect was much smaller because the rest of the evidence was stronger.

The governing Supreme Court framework

Improper argument can justify reversal, but only when its effect on the fairness of the proceeding is serious enough under the applicable standard of review.

First, Chapman v. California establishes that constitutional trial errors, including improper prosecutorial comment on a defendant’s silence, do not automatically require reversal; on direct review, the State must show the error was harmless beyond a reasonable doubt. Chapman is therefore both the leading harmless-error case and a leading prosecutorial-argument case. The Court found the prosecutor’s repeated comments on Ruth Chapman’s failure to testify unconstitutional and concluded that the State had not carried its burden of showing harmlessness. Edwards’s Alabama article uses Chapman as the principal example of an improper prosecutorial argument that did warrant a new trial.

Second, later Supreme Court cases show that improper prosecutorial argument does not always justify relief. In Donnelly v. DeChristoforo, the Court declined to reverse based on an improper remark that, while wrong, did not render the trial fundamentally unfair. In Darden v. Wainwright, the Court again condemned inflammatory prosecutorial remarks but held that the comments, considered in context, did not justify habeas relief. Those cases illustrate the Court’s contextual approach: the question is not whether the argument was proper, but whether it created constitutionally significant prejudice warranting a new trial.

What the scientific research shows

One useful concrete example comes from Edwards’s analysis of Chapman v. California. In that study, respondents were randomly assigned to trial summaries with or without the prosecutor’s improper comments on the defendant’s failure to testify. Edwards then translated the resulting verdict preferences into estimated verdict probabilities for California capital felony trials. He reports that the prosecutor’s comments increased the estimated probability of conviction by .156—a substantial effect consistent with the Supreme Court’s conclusion that the State had not shown harmlessness.

Edwards contrasts Chapman with Lee v. Smeal, a post-conviction case involving improper prosecutorial references to a co-defendant’s incriminating statements during closing argument. Using the same general method, he estimates that the improper argument increased the probability of conviction by only .040. That smaller effect was consistent with the lower court’s refusal to grant relief. The comparison is valuable because it shows why general statements about prosecutorial misconduct are not enough. The same category of error can have very different consequences depending on the surrounding evidence and the posture of the case.

The broader jury literature helps explain why. Devine’s review reports that attorney behavior usually does not generate large direct effects by itself, but arguments can still matter by shaping the framework through which jurors interpret the evidence. That is a useful fit for improper-argument doctrine: the danger is often not that a single remark replaces the evidence, but that it subtly changes how jurors understand the whole case.

Why this issue matters

Improper prosecutorial arguments matter because they test a basic limit of adversarial advocacy. Prosecutors are permitted to argue forcefully, but not to secure convictions through appeals to forbidden inferences, emotional manipulation, or legal misdirection.

For Fair Trial Analysis, this is an important issue because it is a trial error whose effects can be analyzed scientifically. Courts often have to decide whether improper remarks were harmless or harmful without any reliable way to estimate their likely impact on jurors. In the right case, empirical analysis can help replace speculation with a more disciplined assessment of whether the argument actually mattered.