  We analyze high quality, complete stellar catalogs for four young (roughly 1
Myr) and nearby (within ~300 pc) star-forming regions: Taurus, Lupus3, ChaI,
and IC348, which have been previously shown to have stellar groups whose
properties are similar to those of larger clusters such as the ONC. We find
that stars at higher stellar surface densities within a region or belonging to
groups tend to have a relative excess of more massive stars, over a wide range
of masses. We find statistically significant evidence for this result in Taurus
and IC348 as well as the ONC. These differences correspond to having typically
a ~10 - 20% higher mean mass in the more clustered environment. Stars in ChaI
show no evidence for a trend with either surface density or grouped status, and
there are too few stars in Lupus3 to make any definitive interpretation. Models
of clustered star formation do not typically extend to sufficiently low masses
or small group sizes in order for their predictions to be tested but our
results suggest that this regime is important to consider.
