  We present an analysis of the rotation of young stars in the associations
Cepheus OB3b, NGC 2264, NGC 2362 and the Orion Nebula Cluster (ONC). We
discover a correlation between rotation rate and position in a colour-magnitude
diagram (CMD) such that stars which lie above an empirically determined median
pre-main sequence rotate more rapidly than stars which lie below this sequence.
The same correlation is seen, with a high degree of statistical significance,
in each association studied here. If position within the CMD is interpreted as
being due to genuine age spreads within a cluster, then the stars above the
median pre-main sequence would be the youngest stars. This would in turn imply
that the most rapidly rotating stars in an association are the youngest, and
hence those with the largest moments of inertia and highest likelihood of
ongoing accretion. Such a result does not fit naturally into the existing
picture of angular momentum evolution in young stars, where the stars are
braked effectively by their accretion discs until the disc disperses.
  Instead, we argue that, for a given association of young stars, position
within the CMD is not primarily a function of age, but of accretion history. We
show that this hypothesis could explain the correlation we observe between
rotation rate and position within the CMD.
