  We describe the construction and study of an objectively-defined sample of
early-type galaxies in low-density environments. The sample galaxies are
selected from a recently-completed redshift survey using uniform and
readily-quantified isolation criteria, and are drawn from a sky area of ~700
deg^2, to a depth of 7000 k/ms and an apparent magnitude limit of b_J < 16.1.
Their early-type (E/S0) morphologies are confirmed by subsequent CCD imaging.
Five out of the nine sample galaxies show signs of morphological peculiarity
such as tidal debris or blue circumnuclear rings. We confirm that E/S0 galaxies
are rare in low-density regions, accounting for only ~8% of the total galaxy
population in such environments. We present spectroscopic observations of nine
galaxies in the sample, which are used, in conjunction with updated stellar
population models, to investigate star-formation histories. Environmental
effects on early-type galaxy evolution are investigated by comparison with a
sample of Fornax cluster E/S0s. Results from both samples are compared with
predictions from semi-analytic galaxy formation models. The Mg-sig relation of
E/S0s in low-density regions is shown to be indistinguishable from that of the
Fornax sample. Luminosity-weighted stellar ages and metallicities are
determined by considering various combinations of line-indices. At a given
luminosity, the E/S0 galaxies in low-density regions are younger than the E/S0s
in clusters (by ~2-3 Gyr), and also more metal-rich (by ~0.2 dex). We infer
that an anti-correlation of age and metallicity effects is responsible for
maintaining the zero-point of the Mg-sig relation. The youngest galaxies in our
sample show clear morphological signs of interaction. (Abridged)
