  In hierarchical models, where spheroidals are primarily produced via the
merger of disk galaxies, the number of intrinsically red systems at faint
limits will be substantially lower than that expected in "traditional" models
where the bulk of star formation was completed at high redshifts. We analyse
the optical--near-infrared colour distribution of a large flux-limited sample
of field spheroidal galaxies identified morphologically from archival Hubble
Space Telescope data. The $I_{814}-HK'$ colour distribution for a sample
jointly limited at $I_{814}<$23 mag and $HK'<$19.5 mag is used to constrain the
star formation history of elliptical and S0 galaxies. We detect a significant
deficit of intrinsically red spheroids. From limited spectroscopic and deeper
photometric redshift data, we demonstrate that this deficit results from a
strong evolutionary effect. The space density of spheroidals of all colours at
faint limits is lower than expected for a population of constant comoving
density. We conclude that field ellipticals cannot have formed all of their
stars at high redshift, as has been suggested for the bulk of their clustered
counterparts. We compare alternative evolutionary histories with our data,
including an analytical representation of recent models based on the
hierarchical assembly of ellipticals, which provides a better, though not ideal
fit. The accumulation of spectroscopic redshifts for our sample will allow a
quantitative discrimination between models with extended star formation.
