Animals are sometimes sensitive and sometimes indifferent to variability in
amount of reward, I analyzed a large set of published studies to test whet
her these differences in risk-sensitivity may be influenced by the coeffici
ent of variation (s.d./ mean) of the variable reward and by the presence of
empty rewards. The coefficient of variation had a strong effect on the str
ength of risk-sensitivity among studies with nectarivores, both invertebrat
es and vertebrates. The effect was not significant for studies with non-nec
tarivores, but these studies included only a Limited range of coefficients
of variation, and risk-sensitivity in these studies was based on fewer subj
ects per experimental condition than in studies of nectarivores. The presen
ce of empty rewards had no effect on the strength of risk-sensitivity. The
apparent effect of empty rewards seems to be an artifact of the tendency of
distributions that include zero rewards to have larger coefficients of var
iation than distributions that do not include zero rewards. I introduce the
CV model, a simple descriptive model that accounts for a large proportion
of the variability in levels of risk-sensitivity among experiments. My anal
ysis reserves the apparent discrepancies among studies that report risk-sen
sitivity and risk-indifference. It is necessary to control for the magnitud
e of the coefficient of variation when testing risk-sensitivity under diffe
rent conditions (e.g., energy budgets). A successful descriptive model of r
isk-sensitive choice behavior is useful for guiding future research both in
ecology and in the underlying mechanism of cognitive processes.