assume your latter part of your query is about principle given lack of use in u.s., in some jurisdictions as many as 90% of people charged with criminal offenses plead guilty in return for reduced charge and, most often, a less severe penalty (plea bargaining means that most *justice* in u.s. is administrated rather than adjudicated)...of those going to trial, about 50% opt to have their cases heard by a judge, fewer than 10% nationwide of those charged with a criminal offense demand a jury trial...
u.s. supreme court has held that 7th amendment right to jury trial in civil cases does not apply to state courts, u.s. supreme court has also ruled that juries are required only if possible maximum sentence is six months or more...
research indicates that judge and jury decisions are quite similar, although juries tend to be more than lenient than judges and more willing to consider social, in contrast to strictly legal definition of guilt...
re. jury selection/composition, 'jury of one's peers' in early u.s. republic meant those in the community who knew litigants and circumstances of dispute intimately (quite different from today's emphasis on identifying individuals who do not have knowledge of the persons and issues involved in the case), serving on a jury was considered an important citizen responsibility among the more democratically mind of that era...
of course, historical discrimination in jury selection was routine, for example, women in florida were legally prohibited from serving on juries until the mid-20th century...and while the u.s. supreme court ruled back in the 1880s that blacks could not be excluded on racial grounds (_strauder v west va._, if memory serves), we know that such discrimination was common...in fact, it wasn't until 1986 ((_batson v kentucky_) that the u.s. supreme court held that prosecutors may not use their preemptory challenges (rejection without cause) to exclude members based on race...and it wasn't until 1994 (_j.e.b. v alabama) that this was extended to prevent exclusion based on gender...interesting how all of above were in southern states... mh
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