The term "competency" refers to the minimal qualifications someone must have to be a witness. In order to be a witness, a person other than an expert (experts are a special case discussed later in the course), must meet six basic requirements.
1) Take some kind of oath to tell the truth.
2) Have perceived something relevant to the case
3) Remember what he or she perceived
4) Be able to communicate in some sensible way
5) Not already be involved in the trial as a judge, juror or attorney, and
6) Not be disqualified by the Dead Man's Rule.
Not all these requirements are of equal importance. Three never come up:
a) No attorney in his or her right mind is going to call a witness who knows nothing of relevance, has no memory or cannot communicate.
b) No attorney is going to call the presiding judge or a juror as a witness
c) No judge is going to let a witness take the stand without administering the oath.
That leaves five issues of competency that are important:
1) The "witness/advocate rule" is great litigation fun. Ethical rules prohibit a lawyer from serving simultaneously as a witness and an advocate. You subpoena the other side's lawyer to be a witness and then file a motion to disqualify him or her from representing your opponent because of the witness/advocate rule.
2) The dead man's statutes are state laws so obscure they are a favorite of bar examiners. Basically, a live person cannot claim that a dead person owed them money if there's no written evidence of the debt. Otherwise, an estate would soon be depleted by phony claims. There are dozens of exceptions, qualifications, twists and turns. If you are interested in seeing the Indiana Dead Man's Statutes, click here .
3) Whether a witness's drug or alcohol use has rendered the witness unable to communicate sensibly.
4) Whether a young child can understand the oath and communicate in some sensible way. The usual rule is that a child is competent if the child "is sufficiently intelligent to observe, recollect and narrate the facts and has a moral sense of obligation to tell the truth." This means that when small children are involved, the judge and attorneys will have to question the child about his or her ability to accurately describe what the child has seen, and about whether the child understands the difference between true and false, and will tell the truth.
5) And most importantly, whether the witness has personal knowledge of all the facts to which the witness testifies. Along with relevancy, this is one of the two most fundamental rules of evidence. A witness may only testify to matters about which the witness has personal knowledge. Rule 602 says "a witness may not testify to a matter unless evidence is introduced sufficient to support a finding that the witness has personal knowledge of the matter. " That means it is the obligation of the person calling a witness to establish by preliminary questions that the person has personal knowledge of something relevant.