1. Why is it misconduct for a juror “to relate to the other jurors his own personal experience as original evidence of material facts to be considered in their deliberation”?

In HEAD v. HARGRAVE, 105 U.S. (15 Otto) 45, 49–50, 26 L.Ed. 1028, 1030 (1881), a case involving the value of legal services, Justice Field said:

It was the province of the jury to weigh the testimony of the attorneys as to the value of the services, by reference to their nature, the time occupied in their performance, and other attending circumstances, and by applying to it their own experience and knowledge of the character of such services. To direct them to find the value of the services from the testimony of the experts alone, was to say to them that the issue should be determined by the opinions of the attorneys, and not by the exercise of their own judgment of the facts on which those opinions were given. * * * So far from laying aside their own general knowledge and ideas, the jury should have applied that knowledge and those ideas to the matters of fact in evidence in determining the weight to be given to the opinions expressed * * *. While they cannot act in any case upon particular facts material to its disposition resting in their private knowledge, but should be governed by the evidence adduced, they may, and to act intelligently they must, judge of the weight and force of that evidence by their own general knowledge of the subject of inquiry. If, for example, the question were as to the damages sustained by a plaintiff from a fracture of his leg by the carelessness of a defendant, the jury would ill perform their duty and probably come to a wrong conclusion, if, controlled by the testimony of the surgeons, not merely as to the injury inflicted, but as to the damages sustained, they should ignore their own knowledge and experience of the value of a sound limb. Other persons besides professional men have knowledge of the value of professional services; and, while great weight should always be given to the opinions of those familiar with the subject, they are not to be blindly received * * *.