Can a juror be substituted at any time before the jury returns a verdict?

Penal Code section 1089 and Code of Civil Procedure section 233 (former Pen.Code, 1123) specify that a juror may be substituted at any time before the jury returns a verdict if upon "good cause shown to the court [the juror] is found to be unable to perform his duty." ¹⁷ Neither section 1089 nor section 233 define "good cause." It is clear to us, however, that a juror's serious and wilful misconduct is good cause to believe that the juror will not be able to perform his duty. Misconduct raises a presumption of prejudice (People v. Honeycutt (1977) 20 Cal.3d 150, 156, 141 Cal.Rptr. 698, 570 P.2d 1050; People v. Conkling (1896) 111 Cal. 616, 628, 44 P. 314), which unless rebutted will nullify the verdict. The likelihood of the People rebutting that presumption is, of course, far less when the offending juror remains on the jury and participates in the verdict than when the juror is promptly removed. Consequently, "[s]ubstitution of an alternate juror upon a showing of good cause is desirable to maintain judicial efficiency. By means of substitution retrial of lengthy cases may be avoided." (People v. Collins (1976) 17 Cal.3d 687, 692, 131 Cal.Rptr. 782, 552 P.2d 742.)

We recently decided People v. Holloway (1990) 50 Cal.3d 1098, 269 Cal.Rptr. 530,

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