Experiments on nonmonotonic reasoning. The coherence of human probability judgments
Abstract
Nonmonotonic reasoning is often claimed to mimic human common sense reasoning.
Only a few studies, though, investigated this claim empirically. In the present
paper four psychological experiments are reported, that investigate three rules of
system p, namely the and, the left logical equivalence, and the or rule. The
actual inferences of the subjects are compared with the coherent normative upper
and lower probability bounds derived from a non-infinitesimal probability semantics
of system p.
We found a relatively good agreement of human reasoning and principles of
nonmonotonic reasoning according to the coherence interpretation of system p.
Contrary to the results reported in the “heuristics and biases” tradition, the subjects
committed relatively few upper bound violations (conjunction fallacies). More lower
than upper bound violations were observed. When the premises were presented in
terms of intervals higher mean lower bounds were observed as when the premises
were presented in terms of point percentages.