An expert witness, particularly in common law countries such as the United Kingdom, Australia, and the United States, is a person whose opinion by virtue of education, training, certification, skills or experience, is accepted by the judge as an expert. The judge may consider the witness's specialized opinion about evidence or about facts before the court within the expert's area of expertise, to be referred to as an "expert opinion". Expert witnesses may also deliver "expert evidence" within the area of their expertise. Their testimony may be rebutted by testimony from other experts or by other evidence or facts.
In United States federal law, the Daubert standard is a rule of evidence regarding the admissibility of expert witness testimony. A party may raise a Daubert motion, a special motion in limine raised before or during trial, to exclude the presentation of unqualified evidence to the jury. The Daubert trilogy are the three United States Supreme Court cases that articulated the Daubert standard:
Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), is a United States Supreme Court case determining the standard for admitting expert testimony in federal courts. In Daubert, the Court held that the enactment of the Federal Rules of Evidence implicitly overturned the Frye standard; the standard that the Court articulated is referred to as the Daubert standard.
First adopted in 1975, the Federal Rules of Evidence codify the evidence law that applies in United States federal courts. In addition, many states in the United States have either adopted the Federal Rules of Evidence, with or without local variations, or have revised their own evidence rules or codes to at least partially follow the federal rules.
Witness impeachment, in the law of evidence of the United States, is the process of calling into question the credibility of an individual testifying in a trial. The Federal Rules of Evidence contain the rules governing impeachment in US federal courts.
In common law, a foundation is sufficient preliminary evidence of the authenticity and relevance for the admission of material evidence in the form of exhibits or testimony of witnesses. Although the word "Foundation" does not appear in the Federal Rules of Evidence, scholars have argued that its existence is displayed, albeit implicitly, when viewing all the rules in context.
Holmes v. South Carolina, 547 U.S. 319 (2006), was a decision by the United States Supreme Court involving the right of a criminal defendant to present evidence that a third party instead committed the crime. The Court vacated the rape and murder conviction in South Carolina of a man who had been denied the opportunity to present evidence of a third party's guilt, because the trial court believed the prosecutor's forensic evidence was too strong for the defendant's evidence to raise an inference of innocence. The Court ruled unanimously that this exclusion violated the right of a defendant to have a meaningful opportunity to present a complete defense, because the strength of a prosecutor's case had no logical relationship to whether a defendant's evidence was too weak to be admissible.
United States of America v. $124,700 in U.S. Currency, 458 F.3d 822, was a decision of the United States Court of Appeals for the Eighth Circuit that was handed down on August 18, 2006.
Whorton v. Bockting, 549 U.S. 406 (2007), was a United States Supreme Court case about the application of the Confrontation Clause and whether Crawford v. Washington (2006) applied retroactively. Justice Samuel Alito, writing for a unanimous Court, ruled that Crawford did not apply retroactively.
An ultimate issue in criminal law is a legal issue at stake in the prosecution of a crime for which an expert witness is providing testimony.
United States v. Weitzenhoff, 35 F.3d 1275 is a legal opinion from the Ninth Circuit Court of Appeals that addresses the confusing mens rea requirement of a federal environmental law that imposed criminal sanctions on certain polluters. The main significance of the court's opinion was that it interpreted the word "knowingly" in the statute to mean a general awareness of the wrongfulness of one's actions or the likelihood of illegality, rather than an actual knowledge of the statute being violated. Circuit Court Judge Betty Binns Fletcher authored the majority's legal opinion in this case.
Melendez-Diaz v. Massachusetts, 557 U.S. 305 (2009), is a United States Supreme Court case in which the Court held that it was a violation of the Sixth Amendment right of confrontation for a prosecutor to submit a chemical drug test report without the testimony of the person who performed the test. While the court ruled that the then-common practice of submitting these reports without testimony was unconstitutional, it also held that so called "notice-and-demand" statutes are constitutional. A state would not violate the Constitution through a "notice-and-demand" statute by both putting the defendant on notice that the prosecution would submit a chemical drug test report without the testimony of the scientist and also giving the defendant sufficient time to raise an objection.
The Supreme Court of the United States handed down nineteen per curiam opinions during its 2009 term, which began on October 5, 2009, and concluded October 3, 2010.
Giles v. California, 554 U.S. 353 (2008), was a case decided by the Supreme Court of the United States that held that for testimonial statements to be admissible under the forfeiture exception to hearsay, the defendant must have intended to make the witness unavailable for trial.
Tome v. United States, 513 U.S. 150 (1995), was a case decided by the Supreme Court of the United States that held that under Federal Rules of Evidence Rule 801(d)(1)(B), a prior consistent statement is not hearsay only if the statement was made before the motive to fabricate arose.
United States v. Valenzuela-Bernal, 458 U.S. 858 (1982), is a United States Supreme Court case that determined the constitutionality of deporting aliens who might give testimony in criminal alien smuggling prosecutions. Because deporting alien witnesses might take away a testimony that would be both “material and favorable” to the defendant, it gives rise to a potential motion from the defense to dismiss the indictment under the Compulsory Process Clause of the Sixth Amendment and the Due Process Clause of the Fifth Amendment.
The Supreme Court of the United States handed down fourteen per curiam opinions during its 2011 term, which began October 3, 2011 and concluded September 30, 2012.
Glasser v. United States, 315 U.S. 60 (1942), was a landmark decision of the US Supreme Court on two issues of constitutional criminal procedure. Glasser was the first Supreme Court decision to hold that the Assistance of Counsel Clause of the Sixth Amendment required the reversal of a criminal defendant's conviction if his lawyer's representation of him was limited by a conflict of interest.
Chambers v. Mississippi, 410 U.S. 284 (1973), was a United States Supreme Court case in which the Court held that a state may not enforce its rules of evidence, such as rules excluding hearsay, in a fashion that disallows a criminal defendant from presenting reliable exculpatory evidence and thus denies the defendant a fair trial.
Cone v. Bell, 556 U.S. 449 (2009), was a case in which the United States Supreme Court held that a defendant was entitled to a hearing to determine whether prosecutors in his 1982 death penalty trial violated his right to due process by withholding exculpatory evidence. The defendant, Gary Cone, filed a petition for postconviction relief from a 1982 death sentence in which he argued that prosecutors violated his rights to due process under the Fourteenth Amendment by withholding police reports and witness statements that potentially could have shown that his drug addiction affected his behavior. In an opinion written by Justice John Paul Stevens, the Supreme Court held that Cone was entitled to a hearing to determine whether the prosecution's failure to disclose exculpatory evidence violated Cone's right to due process; the Court noted that "the quantity and the quality of the suppressed evidence lends support to Cone’s position at trial that he habitually used excessive amounts of drugs, that his addiction affected his behavior during his crime spree". In 2016, Gary Cone died from natural causes while still sitting on Tennessee's death row.
