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Judicial systems around the world have very different ways of determining guilt in a criminal trial. Within the United States, in order for a defendant to be found guilty, a jury must find him or her guilty beyond a reasonable doubt. Scholars have spent countless hours trying to determine what contributes a jury guilty verdict; however, there is still no magic formula that appears to apply in all cases. Things that may contribute to a guilty verdict include: the amount of evidence presented by the prosecution; the quality of the evidence presented by the prosecution; the experience and believability of the prosecutor trying the case; the defense presented, or lack thereof, by the defendant; and the general characteristics of the defendant.
Within the United States, the prosecution has the burden of proving the defendant guilty beyond a reasonable doubt. The precise definition of "beyond a reasonable doubt" has also been debated by judges, lawyers, and scholars alike for as long as the term has been in existence. Most definitions liken the burden to being convinced to around 95% surety or more. In order for the prosecution to secure a guilty verdict, most jurisdictions require the jury to reach a unanimous verdict, meaning they must all agree to return a not guilty verdict.
The prosecution in a criminal trial will present evidence to the jury in an attempt to convince them of the defendant's guilt. Evidence may be documentary, physical, demonstrative, or testamentary. While simply introducing a large volume of evidence should not, in and of itself, equate to a guilty verdict, juries are often swayed by sheer volume when it comes to evidence of guilt. Jurors are lay people and, as such, are often ill-equipped to sift through the volume of evidence presented to determine how relevant or reliable it may be.
The quality of the evidence presented clearly affects the chance of a jury returning a guilty verdict. For example, in a murder trial, presenting DNA evidence that positively identifies the defendant as being present at the crime scene is compelling evidence of guilt. Likewise, introducing evidence that the murder weapon was found on the defendant will likely go a long way in contributing to a guilty verdict. Expert witnesses can work for, or against, the prosecution. While juries tend to believe their testimony, they can also become very confused by it and may simply discard it altogether if they do not understand it.
Witness testimony can also play a large role in a guilty verdict. While eye witness testimony has been scientifically shown to be less-than-accurate, juries tend to believe it nonetheless. When a victim testifies, his or her testimony will clearly have a powerful impact on the jury.
In the United States, a defendant is not required to present any defense at all. A defendant has a right against self-incrimination in addition to the fact that the prosecution bears the burden of proving guilt, not the defendant proving innocence. While this is the legal standard, juries often want to hear from the defendant and can take it as an admission of guilt of the defendant does not testify, despite the legal requirement that they not do so.
While subjective factors, such as the likability of the prosecutor or the defendant, or the appearance of either one, should not contribute to a guilty verdict, studies show that, in reality, they often do. An experienced prosecutor knows how to engage a jury, which goes a long way in a criminal trial. On the other hand, many defendants simply do not present themselves well in court, which may have a subtle effect on the jury, even if they do not consciously realize it.