At least in North American culture, lowering the pitch of one's voice has relatively predictable consequences. In the 2012 research summarized by Science Daily, summarized by Anderson at Duke, both men and women perceived female politicians with lower pitch to be stronger, more competent and trustworthy. Only males, however, perceived the lower pitch in men as a sign of strength and greater competence. Women apparently saw the lower-pitch male voices as only more trustworthy. Exactly why that happened, the researchers would not speculate, of course, but there are times when advising a student to assume a voice lower or higher in pitch does or does not make sense. The process of helping learners do that often involves haptic or kinaesthetic techniques to establish new awareness of the voice and resonance centers. It is relatively easy, in fact, using body-based procedure to achieve at least temporarily the appearance or feeling of being more confident, what a colleague terms the "Whistle a happy tune" effect. Whether you should explicitly suggest that to students as a strategy, given their respective cultures and interlanguage "identities"-- or even alter your own voice simply to achieve that effect--is another question entirely. In most cases, probably not. Generally, better that the indices of confidence emerge over time, progressively from success and integration of L2 (especially pronunciation) and self, rather than de-contextualized, affective staging. Caveat Emptor . . .