Some MTF transpeople opt for vocal chord surgery (not many, AFAICT, due to the cost and the fact that other techniques work well enough for most people).
From one company that offers it:
Most transgender voice surgery performed is feminizing voice surgery. In order to affect pitch, voice surgery is used to modify the physical characteristics of the vocal cords that determine pitch. These are vocal fold tension, length, and mass
FTM transpeople develop deeper voices just due to taking testosterone, which affects the aforementioned “vocal fold tension, length, and mass.”
So basically men have deeper voices because they have a lot more testosterone than women, so deep voices are likely an accidental of evolution rather than something specifically evolved.
However, it might have been accelerated by sexual selection once it became obvious that deep voices not only indicate masculinity, but adulthood, and in men it seems that appearing definitely not childlike is strongly selected for.
The characteristic pitches of male/female voices are called formants and are caused by resonant filter elements in the vocal tract (as stated by @Francis_Vaughan). These formants are generally associated with vowel sounds, and do not significantly depend of the pitch of the initial vocalisation from the vocal cords. The frequencies of the formants depend primarily on the actual and relative size of the vocal structures, and are impacted during growth and development (as are the vocal cords) by hormones such as testosterone.
The “chipmunk” effect produced when singing is pitched-shifted by speeding up playback is due to both the pitch and the formants being shifted up - a higher set of formants is associated with a smaller vocal tract, moving from male to female to child to impossibly small (like a medium-sized rodent). The ability to isolate formant structure from a pitched vocalisation, modify the pitch, and then reapply the previous formant response is a key operation in digital signal processing, and is required to maintain a natural-sounding result when implementing large pitch-shift using a tool such as Autotune. An alternative operation is also possible - shifting the formant structure independent of the original pitched vocalisation to make a voice change gender or age.
High frequencies reflect off objects more,
low frequencies penetrate through, the “booming” effect. This may give them authority.
Mens ears and brains may be wired to despise the high pitch of children and ladies who are excited, so that they are pushed away and leave them the women and children in peace if they are getting yelled at, and that may be an evolutionary advantage… if it means the women had more children and more children survived to adulthood.
I don’t think anyone is saying that. But we do need to have more than a hunch when we ascribe things to evolution—falsifiable hypotheses are a good starting point. Otherwise, the speaker is just a fabulist.
Well, theorizing about the evolutionary roots of any functional characteristic seems to me unfalsifiable, however a large part of biology research deals with this. Some explanations seem to be more reasonable, some not. It now looks to me that making assumptions about humans is more, how should I put it, culturally suspect. However, If we just see ourselves as another species, I can’t find a convincing reason on why it should be so. Maybe it ain’t, though.
Some hypotheses are much less reasonable than others, though. Like, men despising the voices of women and children is something that would be strongly selected against, not for.
Which is one reason that the castrati had higher voices as they were usually surgically altered prior to puberty, hence the lack of hormone generating glands.
