Abstract
We review evidence for
Macphail’s (1982
,
1985
,
1987)
Null Hypothesis
, that nonhumans animals do not differ either qualitatively or quantitatively in their cognitive capacities. Our review supports the
Null Hypothesis
in so much as there are no qualitative differences among nonhuman vertebrate animals, and any observed differences along the qualitative dimension can be attributed to failures to account for contextual variables. We argue species do differ quantitatively, however, and that the main difference in “intelligence” among animals lies in the degree to which one must account for contextual variables.