If you want a concrete reason that "blood" ties are non-imaginary, there's kin selection: http://en.wikipedia.org/wiki/Kin_selection http://en.wikipedia.org/wik

If you want a concrete reason that "blood" ties are non-imaginary, there's kin selection: http://en.wikipedia.org/wiki/Kin_selection http://en.wikipedia.org/wiki/Kin_selection