Note that the published version of this article is significantly different
from the version on this website.
"We study efficient, Bayes-Nash incentive compatible mechanisms
in a social choice setting that allows for informational and allocative
externalities. We show that such mechanisms exist only if a congruence
condition relating private and social rates of information substitution
is satisfied. If signals are multi-dimensional, the congruence condition
is determined by an integrability constraint, and it can hold only in
nongeneric cases where values are private or a certain symmetry assumption
holds. If signals are one-dimensional, the congruence condition reduces
to a monotonicity constraint and it can be generically satisfied. We
apply the results to the study of multi-object auctions, and we discuss
why such auctions cannot be reduced to one-dimensional models without loss
here for the online version.