  (Abridged) We apply Principal Component Analysis (PCA) to a sample of
early-type galaxies from the Sloan Digital Sky Survey (SDSS) in order to infer
differences in their star formation histories. Out of the first few principal
components (PC), we study four which give information about stellar populations
and velocity dispersion. We construct two parameters (eta and zeta) as linear
combinations of PC1 and PC2. We find zeta to be most sensitive to recent
episodes of star formation, and eta to be strongly dependent on the average age
of the stellar populations. The distribution of the eta component of the
composites appear to be indistinguishable between high and low density regions,
whereas the distribution of zeta parameters have a significant skew towards
lower values for galaxies in low density regions. This result suggests that
galaxies in lower density environments are less likely to present weak episodes
of recent star formation. In contrast, a significant number of galaxies from
our high density subsample -- which includes clusters (both outer regions and
centres) and groups -- underwent small but detectable recent star formation at
high metallicity, in agreement with recent estimates targeting elliptical
galaxies in Hickson Compact Groups and in the field (Ferreras et al.).
