The Evidence That Changed the Case
Sometimes a lawsuit or criminal prosecution turns on hundreds of witnesses and thousands of documents.
Sometimes it turns on one.
A confession hidden from the defense. A recording the jury was allowed to hear. An expert opinion that may—or may not—be scientifically reliable. A collection of digital records quietly created every time a cell phone connects to a tower.
Evidence can change a case because of what it proves.
But some of the most important evidence disputes in American courts have involved a different question:
Should the evidence have been available—or admitted—at all?
That question can be just as consequential as what the evidence actually says.
Over decades, the Supreme Court has confronted cases involving evidence that was withheld, evidence that was scientifically disputed, evidence introduced without the person who made the statement appearing in court, and digital evidence gathered through technology that earlier generations could scarcely have imagined.
The resulting decisions show that evidence is not simply information placed in front of a jury.
Before jurors ever see it, the law may have a great deal to say about where it came from, how reliable it is, whether the opposing side received it, how the government obtained it and whether someone must appear in court to defend it.
The Confession the Defense Never Saw
Few examples demonstrate the importance of a single piece of evidence better than Brady v. Maryland.
John Brady and a companion, Donald Boblit, were prosecuted for murder in Maryland.
Brady admitted participating in the crime but maintained that Boblit had committed the actual killing. Brady's attorney asked prosecutors for Boblit's statements.
Several were provided.
One was not.
In the withheld statement, Boblit admitted committing the homicide.
Brady did not learn about that statement until after he had been tried, convicted and sentenced to death.
The missing statement did not erase Brady's own involvement in the crime. In fact, his lawyer had conceded first-degree murder at trial while asking the jury not to impose capital punishment.
But who had actually committed the killing could matter enormously when the jury considered punishment.
The dispute eventually reached the Supreme Court.
In 1963, the Court established a principle that would become so important that lawyers still refer to this category of evidence simply by Brady's name.
The prosecution violates due process when it suppresses evidence favorable to an accused that is material to guilt or punishment. The rule does not depend on whether the prosecutor acted in good or bad faith.
Brady received relief as to punishment rather than a new determination of guilt.
But the significance of the case extended far beyond him.
It established that a prosecutor's responsibility is not limited to presenting evidence supporting the government's case. Evidence favorable to the accused can itself carry constitutional significance.
The evidence that changes a case, in other words, may be evidence the jury never got to see.
The Scientific Evidence a Judge Must Screen
Thirty years after Brady, another case confronted a completely different evidentiary problem.
This time, everyone knew the evidence existed.
The question was whether the science behind it was reliable enough for a courtroom.
The case was Daubert v. Merrell Dow Pharmaceuticals.
Two children and their parents sued the manufacturer of Bendectin, a prescription anti-nausea medication. They alleged that the mothers' use of the drug during pregnancy caused serious birth defects.
The pharmaceutical company presented an expert who had reviewed more than 30 published studies involving more than 130,000 patients. According to his analysis, the published literature did not establish Bendectin as a risk factor for human birth defects.
The plaintiffs had experts of their own.
Eight experts were prepared to support the plaintiffs' position using sources that included animal studies, chemical analyses and reanalyses of previously published human studies.
The federal district court excluded the plaintiffs' expert evidence and entered summary judgment for the pharmaceutical company. The court of appeals affirmed under the evidentiary standard then being applied.
The Supreme Court took the case.
Its 1993 decision transformed the way federal courts approach expert scientific evidence.
The Court rejected the idea that the old Frye "general acceptance" test controlled under the Federal Rules of Evidence. Instead, the Court said trial judges must make a preliminary assessment of whether proposed scientific expert testimony is both relevant and reliable.
Among the considerations a judge might examine are whether a theory can be and has been tested, whether it has undergone peer review, its known or potential error rate and its acceptance within the relevant scientific community. The inquiry is flexible rather than a rigid checklist.
That makes the judge a kind of evidentiary gatekeeper.
The judge is not supposed to decide which scientist ultimately deserves to win the factual dispute. Cross-examination and competing evidence remain central tools for challenging admissible testimony.
But the court must first decide whether proposed expert testimony has a sufficiently reliable foundation to enter the courtroom in the first place.
That can determine the fate of an entire case.
If a plaintiff must prove that a product caused an injury, for example, but the testimony necessary to establish causation is excluded, there may be nothing left for a jury to decide.
Sometimes the decisive evidence changes the case because it is admitted.
Sometimes it changes the case because the jury never hears it.
The Recording That Confronted the Constitution
Evidence can also be highly persuasive and still present a constitutional problem.
That is what happened in Crawford v. Washington.
Michael Crawford was prosecuted for assault and attempted murder after stabbing a man whom he claimed had tried to rape his wife, Sylvia.
Police interrogated both Michael and Sylvia.
Sylvia's statement was recorded.
When Michael went to trial, however, Sylvia did not testify because Washington's marital privilege prevented the state from compelling her testimony.
Prosecutors nevertheless played her recorded statement for the jury, using it to undermine Michael's claim of self-defense.
That created a constitutional problem.
The Sixth Amendment gives a criminal defendant the right "to be confronted with the witnesses against him."
Michael Crawford's argument was straightforward in principle: the prosecution was using his wife's recorded police statement against him, but he had no opportunity to cross-examine her at trial.
At the time, courts could admit some statements from unavailable witnesses if judges considered them sufficiently reliable under the framework established by an earlier Supreme Court decision.
The Supreme Court changed course.
In 2004, it held that when testimonial statements are involved, the Constitution generally requires that the witness be unavailable and that the defendant previously had an opportunity for cross-examination.
The ruling changed far more than Crawford's case.
It reshaped Confrontation Clause law and forced courts around the country to confront a deceptively difficult question: which out-of-court statements are "testimonial"?
But at the center of the constitutional transformation was something quite ordinary:
a recording.
The recording had not been fabricated. The case did not turn on whether the tape accurately captured Sylvia's words.
The problem was the procedure surrounding its use.
Evidence law sometimes asks whether information is true.
The Constitution can require another question:
Who gets to challenge the person saying it?
Then Evidence Became Data
For most of American legal history, evidence had a physical quality.
A letter.
A weapon.
A photograph.
A fingerprint.
A ledger.
A witness sitting in a chair.
Modern litigation has added something different.
Data.
The change is illustrated vividly by Carpenter v. United States.
Timothy Carpenter was prosecuted in connection with a series of armed robberies. During the investigation, the government obtained records from wireless carriers showing which cell sites Carpenter's phone had connected to.
There were a lot of them.
The government obtained 12,898 location points covering 127 days—an average of about 101 data points per day.
At trial, an FBI agent used the cell-site information to produce maps placing Carpenter's phone near several robbery locations. Prosecutors emphasized the records as corroboration of his presence around the crimes.
But those records presented a question earlier courts had never needed to confront on that scale.
The government had not secretly attached a tracking device to Carpenter.
The location records had been created and stored by his wireless carriers as his phone interacted with the cellular network.
And the government had obtained them under a statutory court-order process requiring less than the probable cause ordinarily required for a search warrant.
Carpenter argued that obtaining such an extensive record of his movements without a warrant violated the Fourth Amendment.
The Supreme Court agreed.
In 2018, the Court held that the government's acquisition of Carpenter's historical cell-site records constituted a search under the Fourth Amendment and that the government generally must obtain a warrant supported by probable cause to acquire that kind of historical location information. The Court emphasized that its ruling was narrow and did not decide every question involving location data or other business records.
The evidence in Carpenter illustrates a fundamental change in what evidence can now be.
A person does not need to write a diary describing where he went.
His devices may create one automatically.
The Evidence That Wasn't Preserved
There is another category of evidence with potentially enormous consequences:
evidence that once existed but was not properly preserved.
Arizona v. Youngblood confronted that problem.
The case involved the sexual assault of a 10-year-old boy. Medical personnel collected biological evidence, and police also collected the child's clothing.
But the clothing was not refrigerated.
Later testing could not identify the assailant. At trial, experts testified that properly preserved samples subjected to timely testing might have produced information capable of exonerating the defendant, Larry Youngblood.
Youngblood was convicted.
An Arizona appellate court reversed, but the U.S. Supreme Court ultimately disagreed with its constitutional analysis.
The Supreme Court distinguished potentially useful evidence from the suppression of known favorable material involved in cases such as Brady. When evidence is only potentially useful, the Court held that failure to preserve it does not violate due process unless the defendant can show bad faith by police.
That rule created a difficult dividing line.
Imagine evidence that could answer the central factual question in a prosecution—but deteriorates before anyone performs the test capable of producing the answer.
What exactly was lost?
Evidence proving innocence?
Evidence confirming guilt?
Nothing useful at all?
Once the material is gone, that question may become impossible to answer.
That is precisely what makes preservation disputes so consequential.
The evidence capable of changing the case may no longer exist in a usable form.
Evidence Does Not Enter a Courtroom Automatically
Taken together, these cases reveal something television courtroom dramas often skip.
Finding evidence is only the beginning.
Before a jury relies upon it, lawyers and judges may have to answer an entirely different set of questions.
Was favorable evidence withheld from the defense?
Is expert testimony sufficiently reliable?
Does admitting a recorded statement violate the defendant's confrontation rights?
Did the government obtain digital information through a constitutionally permissible search?
Was potentially important physical evidence preserved?
And those are only some of the possibilities.
Evidence can be challenged as hearsay. Its authenticity can be disputed. Its relevance may be questioned. Its prejudicial effect may substantially outweigh its legitimate probative value. A privilege may prevent disclosure. A search may be challenged. The chain of custody may be disputed. An expert's methodology may be attacked.
The fact that something exists does not automatically mean a jury will see it.
And the fact that a jury did not see something does not necessarily mean it was unimportant.
The Most Powerful Evidence May Be Invisible
There is an interesting connection between Brady, Daubert, Crawford, Youngblood and Carpenter.
They involve radically different evidence.
A confession.
Scientific expert testimony.
A police-interview recording.
Biological material.
Thousands of digital location records.
Yet each demonstrates that the legal system must decide more than what evidence proves.
It must decide what evidence can legitimately become part of the process used to determine guilt or liability.
That is why some of the most important evidentiary battles happen outside the jury's presence.
A judge may hear arguments about evidence before trial. A constitutional ruling may prevent information from being used. An appellate court may decide years later that evidence should never have been admitted. Or a court may discover that evidence the defense should have received was never disclosed.
By the time jurors enter the courtroom, an enormous amount of legal work may already have determined the universe of information from which they will make their decision.
And sometimes one item changes that universe completely.
A statement that should have been disclosed.
An expert opinion that must first demonstrate reliability.
A recording whose speaker cannot be questioned.
A biological sample that was not preserved.
Or thousands of silent electronic records capable of reconstructing where a person went months earlier.
The evidence that changes a case is not always the dramatic object held up in front of a jury.
Sometimes it is the evidence the jury was never allowed to hear.
And sometimes it is the evidence no one realized existed until technology—or the law—made it impossible to ignore.
