Asset prices are commonly believed to react sensitively
to economic news. Daily experience
seems to support the view that individual asset
prices are influenced by a wide variety of
unanticipated events and that some events have
a more pervasive effect on asset prices than do
others. Consistent with the ability of investors to
diversify, modern financial theory has focused
on pervasive, or "systematic," influences as the
likely source of investment risk.' The general
conclusion of theory is that an additional component
of long-run return is required and obtained
whenever a particular asset is influenced by systematic
economic news and that no extra reward
can be earned by (needlessly) bearing diversifiable
risk.