It's far from perfect. So are human beings. Suppose, just suppose, that a bug was secretly installed in the jury room. My guess is that in 75% of the cases, a mistrial, after verdict, would be warranted. Revealed would be some extrinsic information, not introduced at trial, but improperly considered. And, if this happens, it is rarely, if ever, discovered. Even if it is, the jurors will bond defensibly and deny it. The requirement of an unanimous verdict brings them together as one, and God help the rat.
The defendant is of a minority race or religion. During recesses, throughout the trial, one juror refers to the defendant as "just another damned-------(you fill in the blanks). They're all alike." The rest of the jurors jump on the bandwagon. Generally, religious or racial bias is vehemently denied, yet lurks beneath the surface, alive and well. A human frailty but jurors are human. When deliberations begin, two people are designated as alternate jurors. They do not participate, but rather remain outside the jury room in the event that one of the other twelve is incapacitated. One of the alternates is brooding over the overtones of prejudice which permeated the jury room during trial. After a verdict of guilty is returned, an astute defense attorney dispatches a private investigator to interview the jurors at their homes. Some refuse to discuss the case, others answer questions in the expected way i,e, everything went properly. The alternate juror, however, perhaps motivated by guilt, explains what has been bothering him. He signs an affidavit. A motion for new trial is made. The judge recalls the jurors and questions them individually. As one, they deny the alternate's allegations. It happened but it can't be proved. The guilty verdict stands. This, or something similar, happens more frequently than you dare imagine.
Do you really believe that all jurors obey their instructions and go home each day, refusing to discuss the case with their spouses? That they don't read newspapers or watch T.V. lest they be influenced by something they see? Does a trend exist wherein jurors are reluctant to convict for murder unless all doubt has been obviated by the prosecution? These are questions lacking answers of certainty.
The most a lawyer can do is to examine all the evidence and formulate his "pre-ordained theory of the case." Thereafter, everything he does, from picking a jury to closing argument, is consistent with, and in support of, his theory of the case. His ultimate job is to persuade the jury to adopt this theory.
Figuring out a jury is as impossible as truly understanding the legal definition of "reasonable doubt." It is the truest mystery of all.
An attorney must be fully prepared and become attuned with the mindset of the jury. Remain on that frequency and turn them your way.
What a moment that is, when you rise alongside your client and wait for the verdict to be read. Your mouth is dry and your palms drip with sweat.
It's all up to the jury. The ultimate riddle. But, it'll have to do 'till a better thing comes along.