Speech is always produced with a particular variety of the speaker’s vocal timbre, the latter varying with age and sex. Acoustically, this variety is largely a function of the size of the speaker’s larynx and vocal cavity. Children up to and through the age of puberty have a smaller larynx than do adults. Women have smaller larynxes than men throughout their lives. Thus the vocal timbre of female speech can often be the same as that of children. When adult women’s speech is not full-timbred, it produces the effect of some childishness.
From the point of view of cultural norms, deeper voices generally inspire greater confidence in auditors. Since men have deeper voices than women in virtue of their larger larynxes, their utterances ceteris paribus tend to inspire greater confidence than do those of women. The upshot of these timbre variations is that women’s voices are ill-suited in contexts where what they say needs to be believed or taken as authoritative. That is why it has traditionally been the case in the media for women with deeper voices having been preferred to those with weaker or more child-like voices.
Lately, however, one hears more and more women announcers on National Public Radio and elsewhere that sound child-like. This development can only be explained culturally, perhaps as one upshot of the women’s movement. Thus sounding less like a man has come to be valued as a sign of femininity, not of immaturity or childishness. As long as there are listeners who are used to the older norm defined by deeper vocal timbre as a token of authoritativeness, higher-timbred vocal delivery from women will tend to be taken as undermining the veracity or the authority of their utterances.