  We analyze the stock prices of the S&P market from 1987 until 2012 with the
covariance matrix of the firm returns determined in time windows of several
years. The eigenvector belonging to the leading eigenvalue (market) exhibits in
its long term time dependence a phase transition with an order parameter which
can be interpreted within an agent-based model. From 1995 to 2005 the market is
in an ordered state and after 2005 in a disordered state.
