That's a very interesting statement. Could you share some more information about what events in history or research in "evolutionary biology" suggest this ?
I don't know where to start searching from for this information...
Nothing direct from what I've read but I'm extrapolating from various "decline and fall of X civilization(s)" - all great societies have been through a phase where the society no longer produces enough defenders and the ones alive lack vitality to protect against relatively primitive less technologically sophisticated "barbarians".
Also, what we're seeing happenning in Europe right now on immigration. Japan is a bit weird right now.
I don't know where to start searching from for this information...