Here’s an interesting post by Prof. Miller.
Most states, including Louisiana, have rules of evidence that govern the admissibility of evidence in their courts. Those rules, however, are not highest law of the land and must bow in the face of higher laws. One of those laws is the United States Constitution, and in Crane v. Kentucky, the Supreme Court found that:
"[w]hether rooted directly in the Due Process Clause of the Fourteenth Amendment or in the Compulsory Process or Confrontation Clauses of the Sixth Amendment, the Constitution guarantees criminal defendants a meaningful opportunity to present a complete defense."
Appellate courts have found this right to be violated when trial courts apply state rules of evidence in a fashion that is technically correct but "mechanistic" or arbitrary or disproportionate to the purposes that the rule is designed to serve. In other words, an appellate court cannot simply respond to an appellant’s claim that a trial court violated his right to present a defense by excluding evidence based upon the conclusion that the trial court properly applied a rule of evidence.