  Response latency -- the time taken to initiate or complete an action or task
-- is one of the principal measures used to investigate the mechanisms
subserving human and animal cognitive processes. The right tails of response
latency distributions have received little attention in experimental
psychology. This is because such very long latencies have traditionally been
considered irrelevant for psychological processes, instead, they are expected
to reflect `contingent' neural events unrelated to the experimental question.
Most current theories predict the right tail of response latency distributions
to decrease exponentially. In consequence, current standard practice recommends
discarding very long response latencies as `outliers'. Here, I show that the
right tails of response latency distributions always follow a power-law with a
slope of exactly two. This entails that the very late responses cannot be
considered outliers. Rather they provide crucial information that falsifies
most current theories of cognitive processing with respect to their exponential
tail predictions. This exponent constitutes a fundamental constant of the
cognitive system that groups behavioral measures with a variety of physical
phenomena.
