So overall we would expect a fairly wide distribution of states within any population. Due to these conflicting considerations, most complex systems have a spread of properties. This dynamic human organisation is very different from the uniformity assumptions of reversible physics, but does contain aspects of that world also. Hence we are able at times to make simplifying assumptions and treat certain properties of our world as capable of being modelled by reversible equations. The ability to do this however should not blind us to the need to keep in mind that such cases are merely a small sub-set of the possible behaviours of our complex system.
Life is richer than non-life, society richer than biology.
As we climb the complexity ladder we also encounter nested levels of detail, emergent properties that can have very different characteristics to the parts that make them up. What is improbable for a thousand particles may be fairly probable for a few, and what is improbable for unconstrained particles may be very probable for constrained ones. Most work in physics deals with either small numbers of particles or with very large numbers. In the first case individual reversibility can be achieved in constrained systems if we ignore minor disturbances and use short time scales - we smooth out the inherent irreversibility by using simplified formulae. Hence in complexity studies whether the system is of reversible or irreversible form must be contextual and there is no resultant conflict between these ideas.