What is the test for diminished capacity under the felony-murder doctrine?

'In the circumstances of the instant case this interpretation would have substantially eviscerated the defense, which was based upon principles of diminished capacity. Although specific intent to commit the underlying felony is necessary to the operation of the felony-murder doctrine (citation)--so that it is arguable that a defense of diminished capacity would not be entirely unavailable since it could be directed to the issue of intent to commit the underlying felony--nevertheless it is clear that the applicability of such evidence to that narrow issue would be in no way equivalent or comparable to the applicability of such evidence to the broad issue of malice aforethought in the charged offense.'

The language above quoted from People v. Ford was not referred to, and that case was not overruled.