Can a jury impeach its own verdict?

Running side by side with the rule just discussed is the rule that a jury (with exceptions to be noted) may not impeach its own verdict. Since self-accusation and impeachment by fellow jurors may very frequently be the only means by which misconduct of a juror can be determined, it will readily be seen that the two rules do not pull well in double harness. A late expression of the rule against a jury impeaching its own verdict is in Kollert v. Cundiff, 50 Cal.2d 768, 329 P.2d 897. There, in a personal injury action also involving a collision at an intersection, an issue involved the change intervals of an intersection traffic light. After a nine to three defense verdict, on motion for new trial, affidavits of the three dissenting jurors and of one majority juror were offered. According to these affidavits (which also related other matters regarding deliberations in the jury room) the foreman of the jury, not one of the affiants, had told the jurors they would have to abide by the verdict forms completed by him or they would never again be allowed to act as jurors. It also appeared that the foreman, during a court recess, had 'investigated the traffic lights at the intersection where the accident occurred, and his report on the periods of the signals was considered by the jury.' The jurors giving affidavits had, no doubt, received this information from the foreman himself during the jury's deliberations.