A stimulus is a reliable signal of an outcome when the probability that the outcome occurs in its presence is different
from in its absence. Reliable signals of important outcomes are responsible for triggering critical anticipatory or preparatory
behavior, which is any form of behavior that prepares the organism to receive a biologically significant event.
Previous research has shown that humans and other animals prepare more for outcomes that occur in the presence of
highly reliable (i.e., highly contingent) signals, that is, those for which that difference is larger. However, it seems reasonable
to expect that, all other things being equal, the probability with which the outcome follows the signal should also
affect preparatory behavior. In the present experiment with humans, we used two signals. They were differentially followed
by the outcome, but they were equally (and relatively weakly) reliable. The dependent variable was preparatory
behavior in a Martians video game. Participants prepared more for the outcome (a Martians’ invasion) when the outcome
was most probable. These results indicate that the probability of the outcome can bias preparatory behavior to
occur with different intensities despite identical outcome signaling.