  A generic prediction of hierarchical gravitational clustering models is that
the distribution of halo formation times should depend relatively strongly on
halo mass, massive haloes forming more recently, and depend only weakly, if at
all, on the large scale environment of the haloes. We present a novel test of
this assumption which uses a statistic that proves to be particularly
well-suited to detecting and quantifying weak correlations with environment. We
find that close pairs of haloes form at slightly higher redshifts than do more
widely separated halo pairs, suggesting that haloes in dense regions form at
slightly earlier times than do haloes of the same mass in less dense regions.
The environmental trends we find are useful for models which relate the
properties of galaxies to the formation histories of the haloes which surround
them.
