  We present a new interpretation for reduced density matrices of secondary
variables in relativistic systems via an analysis of Wigner's method to
construct the irreducible unitary representations of the Poincar\'e group. We
argue that the usual partial trace method used to obtain spin reduced matrices
is not fully rigorous, however, employing our interpretation, similar effective
reduced density matrices can be constructed. In addition, we show that our
proposal is more useful than the usual one since we are not restricted only to
the reduced density matrices that could be obtained by the ordinary partial
trace method.
