If you're familiar with registrational approaches to teaching the voice, you'll know that a lot of it is to do with balancing two registers or "voices", the chest voice and the head voice. In a rough sense this refers to muscle activation, and generally people will have too much activation of one or another set of opposing muscles, which need to be balanced for people to get from the bottom to the top of their voices and back down again with ease.
There is a common conception among teachers in this field, that generally the majority of women will be of the predisposition to be lacking chest voice, and that many men will lack head voice. What this means in plain english is that the things associated with chest voice (lower notes, volume, projection, stability) will often be lacking in women and that things associated with the head voice (release, ease, higher notes), will be more challenging for men. And indeed, to some degree I've witnessed this in my own students with many women needing to strengthen their chest voices.
Why is it that many women have weaker-sounding voices you ask? Well in a way it has a lot to do with the way men are physically built (with larger muscles and physically bigger instruments), and the fact that their speaking voice (pitch-wise) is often further down - meaning they're spending more time at the bottom of chest voice and are more used to this register. But there might also another reason...
One of the primary roles of the voice teacher is to try to reverse muscle atrophy - to strengthen or reactivate muscles that exist in people but maybe haven't been used in a while and have consequently become weakened, this weakness causing what some call a "collapse of the instrument". So if women often present with weaker chest voice muscles, it could also be said that women already might not, on a daily basis, do those things (other than just simply phonating at a lower pitch) associated with a strong chest voice as often, as men - being louder, prouder, and using a more projected voice.
This connect between someone's psychology (and consequently their ability to speak confidently and with volume) and the strength of their vocal musculature is something that would need to be studied further to be fully scientifically verified, however common sense and experience dictate that there will be some cases in which this is a factor. Which must lead us to question, "Are women chronically less able to express themselves with pride and a sense of worth, than men?" "Are women disempowered in their daily lives on a sociological level to the point where their voices are more likely to become physically weaker?". To some these questions may seem over-dramatic. But it is plausible that this field of enquiry should be examined further and taken with some level of seriousness. Of course there will always be purely physical factors associated with muscle usage and health (ie. "I was born this way"), but psychological and sociological factors are important as well - and moreover are potentially factors that might be changed over time, as women's roles within societies and communities also continue to change.