In 2 experiments, trained military observers identified vehicles in in
frared (thermal) imagery that varied in distance, signal-to-noise rati
o, and orientation. A measure of shape similarity was derived from a c
ontingency tree that allowed prediction of the confusion rates between
any 2 vehicles on the basis of the number of detectable, distinguishi
ng parts. The mean confusion rates between pairs of vehicles were stro
ngly correlated with the nodal distance between these vehicles in the
similarity trees, even though the similarity trees had been constructe
d without knowledge of the confusion rates. Such trees offer the possi
bility for substantial improvements in the modeling of human object id
entification and, when incorporated into training programs, offer a hi
gh potential for reducing the likelihood of identification errors.