What is the test for making a prediction of a defendant's potential future violence?

As the authorities explain, there are a variety of reasons why such consideration is improper. The first and perhaps most obvious problem is the speculative nature of the inquiry that the instruction invites. One principal difficulty, of course, lies in attempting to predict what a particular defendant is likely to be like some 10, 15, 20 or more years in the future when commutation may be considered. In People v. Murtishaw (1981) 29 Cal.3d 733, 767-775, 175 Cal.Rptr. 738, 631 P.2d 446, we reviewed the recent scientific studies demonstrating the general unreliability of attempts to forecast future violence and concluded that in general expert testimony of a defendant's alleged future dangerousness is not sufficiently reliable to be even considered at the penalty phase of a capital trial.