Where I practice in Illinois, the State usually discloses to defense counsel all the evidence it has in a criminal case--the good, the bad, and the neutral.
But the Constitution requires the State to disclose less than everything. The Supreme Court held in Brady v. Maryland "that the suppression by the prosecution of evidence favorable to an accused upon request violates due process where the evidence is material either to guilt or to punishment, irrespective of the good faith or bad faith of the prosecution." (Emphasis added.) Brady v. Maryland, 373 U.S. 83, 87, 83 S. Ct. 1194, 1196-97.
What constitutes evidence "favorable to an accused?" More to the point, who gets to make that call?
I don't like the idea of a prosecutor deciding for me whether or not certain evidence is favorable to my client. Frankly, I think I'm more creative than a lot of prosecutors. What a prosecutor might see as neutral evidence, or evidence that hurts my client, I might see as evidence that proves how poorly the police conducted their investigation.
Let's say the prosecutor discovers what he believes to be surveillance camera footage of my client's red Dodge pickup truck at the scene of a burglary. Obviously, from the prosecutor's perspective, disclosure of this evidence is not required under Brady because the evidence isn't favorable to the accused. But what if the prosecutor failed to recognized that my client's red Dodge pickup truck has a noticeable dent in the rear panel, which is not present on the truck in the surveillance video? Favorable is in the eye of the beholder.
The clear problem with Brady is that it requires the prosecutor to make the completely subjective determination of whether certain evidence is favorable to the accused. But the prosecutor's perspective is calibrated in the opposite direction; he is conditioned to see the inculpatory characteristics of evidence, not the exculpatory characteristics. And the opposite is true of defense attorneys. It's like telling a Bears fan to identify the most lovable Green Bay Packer. Or asking a cat lady to pick you out the perfect dog. Wooof. No thanks.
Brady's importance is largely offset by state and local discovery rules. I'm never really fearful that the State has withheld evidence that I might have found valuable. It just doesn't happen in my neck of the woods (I don't think). But if that were to change, I would have serious problems with the notion of a prosecutor deciding for me which evidence is favorable to my client.