I am often asked why I think most modern mental mediums are female. On our research team, 90% of the mediums are female.
I can only conjecture about this gender discrepancy, but it might have something to do with the evolution-based predisposition female brains have for communication skills.
Alternatively---or additionally and probably more likely---it might simply be a social issue: it is more socially acceptable to be a medium if you are female. Just like men are not necessarily better scientists but it has just been more acceptable historically and socially for men to pursue science, women are not necessarily better mediums but it has just been more socially acceptable for women to practice mediumship. Historically, women with scientific proclivities and men with mediumistic abilities were often told by society to focus their energies elsewhere so those occupations have been dominated by those genders. However, I know there's nothing about science that necessitates an Adam's apple (because I don't have one and I rock at it) and I don't think there's anything about mediumship that necessitates two X chromosomes (because the other 10% on our team are equally as capable as the ladies).
Again, this is all pure speculation and only research will be able to sort it out.