When a male and female singer sing a duet together they have to sing in the same scale, and if they sing in the male scale (say C#3) the female has to sing either in C#3, which is too low, or C#4 which is too high for the female singer as her naormal scale is around G#3 or A#3. If they sing in the female scale (say A#) the male has to sing in A#3, which is too high, or A#2 which is too low as the normal scale of the male siger is C#3 or D#3. Hence they sing in a scale which is in between the two, say F#3.
I have taken the song “Deewana mastana hua dil” from the film Bambai ka Babu (1960) with music by S.D. Burman. The singers are Mohammed Rafi and Asha Bhosale, and the song is in the F (Safed 4) scale. Here is a clip with a sustained mid-Sa followed by an upper-Sa by Rafi, and then a mid-Sa by Asha Bhosale followed by the upper-Sa.
And here are the spectrum plots for the four sections.
In this song Rafi’s mid-Sa is at at F3 (174Hz) and Upper-Sa is at F4 (348Hz), whereas Asha Bhosale’s mid Sa is at F4 and Upper Sa is at F5. Notice that in her Upper-Sa there are peaks at F4 and F5 that are almost equal in amplitude, but the one at F5 is a little higher. This illustrates the rich harmonic cotent in her voice.
To summarise, female singers sing in a higher scale than men because of the anatomy of their vocal chords. Any feedback or views contrary to the above are welcome.