the universe. The cosmological state is essentially an eigenstate of the operators representing those processes, and each such state is orthogonal to the rest. It is only necessary to consider one of these cosmological states to be the “real” history of the actual universe, though, of course, we have only very partial information on which history this actually is. Nevertheless, it is shown in [24] that, if this “real” history is selected with a probability determined by the normal Born rule for the cosmological states, then, from within the history, all observations made at the classical level can be expected to be distributed according to the statistical rules of quantum theory. Two important topics were not addressed in [24], however. First, no detailed model was given of how a measuring apparatus, as a part of the universe being measured, becomes correlated with some other part of the universe and itself decoheres into non-interfering states. Second, no justification of Born’s rule was given for cosmological states; it was simply shown that the assumption of Born’s rule for cosmological states leads to the proper statistical distribution for repeated experiment results (avoiding the circularity problem with some previous arguments based on relative frequencies [27, 28]). In this paper, I will address both of these topics (though, for the present paper, I will take a somewhat restricted view of what a “subsystem” is, as described in Sec. IV). In doing so, it is important to not implicitly presuppose the results of [24], but, rather, to independently support the assumptions made there, in order not to re-introduce circularity problems. As noted in [24], the arguments of Zurek on environment-induced superselection ( einselection ) and entanglement-assisted invariance ( envariance ) are particularly relevant in this regard. Zurek has written extensively on einselection and envariance in the non-relativistic con- text (see, for example, [29–34]). Here, I extend these concepts to the context of a relativistic, spacetime formalism for quantum histories. Einselection addresses the first of the topics in- troduced above, while envariance addresses the second. Section II provides a brief overview of the the consistent histories approach to non- relativistic quantum mechanics and motivates the relativistic generalization developed sub- sequently. This generalization is grounded in the spacetime path formalism, but the math- ematics of path integration is not actually required for the results discussed in this paper. The underlying mathematics can instead be packaged in the familiar notations of relativis- tic states and fields. However, the traditional quantum field theory formalism still presents some difficulties for a straightforward description of relativistic quantum histories. Sec- 3