Whether Witness Competent a Question for Court in Limine.—After the expert is placed upon the stand, as we have seen, the counsel upon the side of the case by which he is summoned interrogates him as to his capacity, the purpose of the interrogation being that his answers shall qualify him and show him to be an expert. Whether or not he is an expert so as to permit the giving of his opinion as part of the case to go to the jury, is for the court to decide in limine, that is, at the threshold, and as a matter of discretion, and the exercise of that discretion, if fair and reasonable, will not be disturbed upon appeal by the higher court. It is permissible, but also discretionary, after the counsel calls the witness and has apparently qualified him, for the counsel upon the other side to cross-examine the witness as to his qualifications before he is examined in chief, with a view of determining whether or not there are limitations upon those qualifications which should prevent the court from permitting him to testify as an expert. The general rule is as stated by Greenleaf in his work on Evidence, Sec. 440, that it is not necessary that the medical expert should have actually practised his profession. Nor is it essential that the witness should belong to any particular school of medicine. The law does not undertake to pass upon conflicting theories of medical practice, in determining the question of the qualification of a medical expert. It is proper, however, for counsel to inquire as to what school of medicine the witness is an adherent, because of its importance in weighing the value of his testimony after it has been given.
Persons Not Duly Licensed Sometimes Held Not Competent.—It has also been a mooted question in those States where it is necessary, in order to enable a person to practise physic or surgery, that he should be licensed, whether a person practising without a license, however extensive his reading and practice, would be considered qualified as an expert witness in a court of justice. This point, so far as diligent examination discloses, has not been determined in any reported case, although it has been suggested at nisi prius and has been, in one instance within the knowledge of the writer, decided that he is not to be considered an expert in matters involving medical knowledge and skill. The reasoning of the court was that the policy of the State is to prohibit persons not possessing the qualifications required to obtain a license, from acting in any capacity as professors and practitioners of medicine or surgery. If the witness is a member of the profession, legally qualified as such, it has been held that he is sufficiently qualified as an expert if he shows that he possesses the average ability of members of his profession. Hall v. Costello, 48 N. H., 176; Tellis v. Kidd, 12 Ala., 648; Wharton on Evidence, Sec. 446; Rogers on Expert Testimony, Secs. 17 and 18; Slocovich v. Orient Mutual Ins. Co., 108 N. Y., 56.
As to the question whether it is necessary that the witness should actually have practised his profession, see the last-cited text-writer, Secs. 43 and 44, who seems to have entertained views opposite to those stated by Professor Greenleaf.
Wharton on Evidence, Sec. 439, states the rule as follows: “He must have special, practical acquaintance with the immediate line of inquiry more than a mere vague, superficial knowledge. But he need not be acquainted with the differentia of the specific specialty under consideration.... A general knowledge of the department to which the specialty belongs would seem to be sufficient.”
Interested Persons may still Testify as Experts.—Since the law forbidding interested persons from being witnesses has been changed, it has been suggested that an interested person although otherwise qualified might not be a competent witness to give an opinion as an expert. But the established doctrine is that he may give such an opinion; the weight of it, however, would be for the jury to determine. Greenleaf on Evidence, Redfield’s edition, Sec. 440, citing Lockwood v. Lockwood, 2 Curtis, 309; Dillon v. Dillon, 3 Curtis, 96, 102. See also Dickinson v. Fitchburg, 13 Gray, 546.
Testimony of Expert, how Impeached.—Sometimes, on cross-examination or otherwise, the fact becomes known that the witness who is proposed as an expert has expressed an opinion on the subject in hand contrary to that which he has given upon the witness-stand, and the question has been raised as to establishing that fact at the outset and before his testimony goes before the jury, in order to enable the trial judge to determine whether he is competent. The rule in that case is that the testimony as to his prior expression of opinion is not to be received at that time, but will come properly up as rebuttal, he having been asked upon his cross-examination, giving time and place, whether he has made the statements attributed to him. An expert witness may in other respects be impeached like any other witness, that is, by the oaths of persons who know him and have known his reputation, and who testify that his reputation for truth and veracity is bad and that they would not believe him under oath. He may also be impeached by producing witnesses to prove that his special knowledge or technical skill is not reliable or adequate to the undertaking which he has assumed. But this testimony must be from personal knowledge of the man and not from general reputation. Wharton on Evidence, Sec. 437; Le Rose v. Commonwealth, 84 Pa. St., 200.[183]
General Rule as to Required Amount of Skill and Experience Stated.—The general rule may be stated thus, as derived from these and other authorities:
The extent of the previous study and investigation, and the amount of skill and information which must be shown, will depend upon the facts of each particular case. But some special and peculiar knowledge or skill must be established, the amount of it to be determined by the trial judge in his discretion. The possession of such knowledge and skill is presumed in medico-legal cases if the witness is a licensed practitioner.
Some Practical Suggestions as to Conduct of Witnesses on the Stand.—In this preliminary examination, the conduct and demeanor of the witness are of no little importance, because it is then and there that he makes his first impression upon the court and jury. He should be perfectly open and unreserved in stating his means of special information, in explaining what are the limits of his personal experience and the extent of his reading; but, at the same time, it would be well for him to avoid all appearance of self-glorification and all tendency to exaggerate his individual acquirements. Often has it occurred that expert witnesses of undoubted capacity and honesty, who are unfortunately grandiose and self-assertive in their manner, have, however honest and able they might be, lost entirely their weight with the court and jury by undue self-complacency and exaggeration of their personal qualifications, during their preliminary examination. This is a matter requiring tact and judgment and nerve, and should be fully understood between counsel calling him and the witness, before the witness is placed upon the stand. In that event, it will be quite safe for the witness to closely follow the questions of counsel by his answers, and to volunteer little or nothing. If his answers are not full and complete enough, counsel can renew the question in the same or in other form or carry the matter into greater detail. If, on the other hand, his answer is too full or he appears too eager, he may create a prejudice against him which nothing can overthrow, and which the art of counsel upon the other side in cross-examination and in making comments upon his testimony when summing up before the jury, will very effectually use to destroy his weight as an expert.[184]