Following on from the Ruby Wax thread, which included some discussion of whether mothers were inherantly better than fathers at childcare, I found this article, which might interest those who were asking for (but failed to get) any evidence based answers as to why there was such a strong belief that mothers were best.
It's quite long and technical, but shows that fathers give different care than mothers, although interestingly not in babyhood.
www.bristol.ac.uk/cmpo/workingpapers/wp175.pdf
There's also a more readable summary here:
www.equalityhumanrights.com/en/newsandcomment/speakerscorner/Pages/Progressiveparenting.aspx