In a book by two psychologists called Mistakes Were Made (but not by me!): why we justify foolish beliefs, bad decisions and hurtful acts, the authors detail how and why prosecutors insist on guilt even when they find overwhelming evidence of innocence. Deceptive blinders, tunnel vision and self justification create a situation where a prosecutor believing himself to be good couldn't possibly be the kind of person who sends the wrong man to prison and ignores all evidence that contradicts that assessment.
Law professors also delved into the problem in an essay called The Multiple Dimensions of Tunnel Vision in Criminal Cases noting that over 170 people convicted of heinous crimes have been proven innocent by DNA evidence since 1990, but that hundreds more have been exonerated over that time period with other evidence establishing that the criminal justice system fails to accurately determine guilt. Even in preliminary stages of criminal cases rather than merely accumulating only the evidence required to convict, prosecutors should also be looking at contradictory evidence of innocence.
The American Bar Association sets forth the obligations for any prosecutor who learns of "new, credible and material evidence creating a reasonable likelihood that a convicted defendant did not commit an offense of which the defendant was convicted." The prosecutor shall:
(1) promptly disclose that evidence to an appropriate court or authority, and
(2) if the conviction was obtained in the prosecutor’s jurisdiction,
(i) promptly disclose that evidence to the defendant unless a court authorizes delay, and
(ii) undertake further investigation, or make reasonable efforts to cause an investigation, to determine whether the defendant was convicted of an offense that the defendant did not commit.
(h) When a prosecutor knows of clear and convincing evidence establishing that a defendant in the prosecutor’s jurisdiction was convicted of an offense that the defendant did not commit, the prosecutor shall seek to remedy the conviction.
The requirements for prosecutors are clear. Seek justice promptly especially if the initial conviction was in error. The solution to the psychological problem of prosecutor's tunnel vision and of not wanting to admit mistakes is to punish the overzealous prosecutors. Those prosecutors who fail in their obligations to seek justice promptly should not be fired, but should also be stripped of their law licenses.
Too often prosecutors acting in bad faith betray the criminal justice system by misusing their authority by bullying innocent defendants into changing pleas with threats and additional criminal charges or by ignoring important evidence of innocence for those already falsely convicted.