As you may know, when you are charged with a crime, you have no obligation to present evidence in your own defense. This is known as the presumption of innocence, and it is the cornerstone of our criminal justice system.
However, in some instances, it may be beneficial to present evidence to a jury that can help create a reasonable doubt. This is true in cases that involve scientific evidence, such as DUI (breath test results), sexual battery (DNA analysis), and burglary (fingerprints).
Accused persons may retain expert witnesses to testify at trial. An expert witness is skilled in a particular field and can provide invaluable insight that may rebut a claim made by the State.
For instance, if you are charged with DUI and the State introduces evidence that your breath-alcohol content was above the legal limit of .08, you may present expert testimony to show that the Intoxilyzer machine was not functioning properly on the date of the breath test.
Experts in breath testing may be former police officers who may have held breath test operator permits. Given their level of training and experience, it is possible that their testimony can help to create doubt in the minds of the jurors as to whether the supposed breath reading is accurate.
Also in DUI cases, toxicologists can be helpful to the defense in showing that an accused person was not legally impaired at the time of the arrest. This is because alcohol is absorbed and metabolized by the body at difference rates. If you are driving at 5:00 p.m. and you are pulled over, you may only be at a .05 at that point. If your body is the absorbtion phase, you may blow a .09 when you submit to the breath test 90 minutes later.
The law prohibits driving while impaired. Your blood-alcohol content cannot be at a .08 or higher at the time when you are driving the car. A toxicologist uses a scientific method known as extrapolation to demonstrate what your true blood-alcohol content would have been at the time you were driving.
Experts may also be helpful in cases with physical evidence, such as DNA or fingerprints. This is because experts may be able to show that the DNA sample was tainted, which could yield false results. A fingerprint expert can demonstrate that the prints were not lifted properly, thus were smudged and could not be used for comparison.
An expert witness should be educated in their field. They should be respected and unbiased. That is, you should consult with your South Florida criminal defense attorney when selecting an expert witness. You should pick somebody who does not have a reputation as a "defense expert." That is, an expert whose income is dependent upon being hired by criminal defense attorneys. In turn, these experts will almost always testify favorably for the defense.
Your expert witness should be respected by the State Attorney's Office as well as the defense bar. He or she should have no professional discipline matters on their record. They should be published (if applicable) and have advanced degrees. If the expert is not necessarily a scientific expert, they should have adequate professional experience and accolades. The expert should be a member of a professional organization.
Expert witnesses can be costly, so make sure that your budget allows you to retain one. If you cannot afford an expert, your attorney may be able to file a motion asking the Court to have the State pay the witnesses fees. Your not likely to get the State to pay for your $500-an-hour DUI expert, but they may pay for mitigation specialists, such as mental health professionals, for purposes of a downward departure hearing.
Not all cases require expert testimony. But every now and again, it is not a bad idea to give a jury an intelligent opinion which may impact their decision.