Academy and Emmy Award winning technologist. Co-founder of Avid Technology. Holder of nearly 60 issued U.S. patents, mostly in media technology.
Research done at Cornell University all the way back in the sixties found that each human voice basically consists of two parts. One part carries most of the speech information, and occupies practically the same frequency band for all speakers. The second part consists of gender and status markers, as well as vowel sounds. This second part is often referred to as voice formants. For example, men have several formants (roughly "resonances" or energy peaks) below the speech information band, while women have several above. These formants mostly carry information about who we are and the state we are in, and not much about the actual words we are saying. It's the reason we say men have lower voices and women have higher voices, when in fact we all communicate using roughly the same band of frequencies. The lower or higher formant frequencies are saying "I am male" or "I am female", among other things.
If you need convincing that we all communicate in the same band of frequencies, think of how difficult it is to understand the words a soprano is singing. In the case of a soprano, she has learned to actually move her speech information to higher frequencies, where most of us are not used to hearing and interpreting it, and it makes it difficult, if not impossible, to understand.
I believe this knowledge has implications for mixing conversations. If you wish to make the information content (words) clearer, apply the boost and cut suggested in this article to roughly the telephone band of frequencies, from 300 to roughly 3300 Hz, for any speaker. This range was carefully chosen for telephones based on considerable research into creating the clearest conversations while using the least resources in the telephone network. The subtle changes suggested in this article won't make the conversation sound like a telephone; it will simply increase the clarity.
I believe that if, on the other hand, your goal is to emphasize the virility or femininity (and perhaps other status--think Darth Vader/James Earl Jones) of a voice, rather than the words being said, apply the boost and corresponding cut to the formants below (for men) and above (for women) this "telephone" range of frequencies.
Just an aside: Telephones don't actually carry the formants of male voices, these formants are all well below 300 Hz, starting at around 85 Hz; yet we still know immediately when a speaker on the phone is male. It is because the telephone does carry the pattern of the overtones of the formants, and the ear/brain is excellent at recreating the fundamental frequencies from that pattern. Experiments show we actually hear a fundamental, a formant, even when it is not physically present, but if only the overtones are present. It is an audible illusion.