  For the first time, the consequences of combining two well-established
empirical relations, describing different aspects of the spectral evolution of
observed gamma-ray burst (GRB) pulses, are explored. These empirical relations
are: i) the hardness-intensity correlation, and ii) the hardness-photon fluence
correlation. From these we find a self-consistent, quantitative, and compact
description for the temporal evolution of pulse decay phases within a GRB light
curve. In particular, we show that in the case of the two empirical relations
both being valid, the instantaneous photon flux (intensity) must behave as
1/(1+ t/\tau) where \tau is a time constant that can be expressed in terms of
the parameters of the two empirical relations. The time evolution is fully
defined by two initial constants, and two parameters. We study a complete
sample of 83 bright GRB pulses observed by the Compton Gamma-Ray Observatory
and identify a major subgroup of GRB pulses (~45 %), which satisfy the
spectral-temporal behavior described above. In particular, the decay phase
follows a reciprocal law in time. It is unclear what physics causes such a
decay phase.
