It's easy to think that all experts dress in pinstriped suits, but that's far from the case. Those who testify as experts in court are simply specialists in a field -- and that could be caring for dolphins or examining gems. Before expert testimony is allowed, however, the attorney presenting the expert must establish to the court's satisfaction that she has a specialized knowledge that can assist the judge or jury on a complex issue. This is called "laying the foundation for expert testimony."
Lay Witness Testimony
When you hear that someone witnessed a crime, you probably imagine a passerby watching the thief snatch a purse or Maxwell bring that silver hammer down on someone's head. It is true that normal witnesses, or lay witnesses, can only testify in court about things about which they have personal knowledge. They must have seen, heard, felt, tasted or touched something that is relevant to an issue in the case. A lay witness cannot give an opinion on matters outside her personal knowledge or base her testimony on what others saw. For example, she can testify that she saw the neighbor with small plastic bags of white powder, but she cannot conclude that the white powder was cocaine.
If a witness is qualified as an expert, however, she can give her opinion about something, even the ultimate question in the case, like whether Maxwell's hammer caused the death. An expert can also explain complex matters within her area of special expertise to a jury. For example, an expert can testify about how the body reacts to loss of blood or how airplane black boxes work.
Qualifying the Expert
Because the scope of expert testimony is so much broader than a lay witness, courts require that the judge rule on expert qualifications before the testimony is heard. In other words, a lawyer must prove to the judge that the expert has the training, education, experience or background that qualifies her to offer specialized opinion testimony. This doesn't always mean the witness must have graduate degrees. Extensive experience as a beekeeper might go farther to qualifying someone as an expert on why bees swarm than someone who has a Ph.D. degree in the sciences.
Voir Dire Testimony
A lawyer "lays the foundation" for expert witness testimony by asking the witness questions that establish her competency and knowledge in the specific field of inquiry. In some cases, this means that the attorney asks questions about her education and published work. In others, the focus might be on experience. After the attorney is done, the opposing attorney has a chance to ask questions that may disprove the expert's qualifications or experience. This cross-examination by the opposing side is known as "voir dire." When voir dire is completed, the judge rules on whether a proper foundation has been laid for the witness to testify as an expert.