  The observed delay in the arrival times between high and low energy photons
in gamma-ray bursts (GRBs) has been shown by Norris et al. to be correlated to
the absolute luminosity of a GRB. Despite the apparent importance of this
spectral lag, there has yet to be a full explanation of its origin. We put
forth that the lag is directly due to the evolution of the GRB spectra. In
particular, as the energy at which the GRB's $\nu F_{\nu}$ spectra is a maximum
($E_{pk}$) decays through the four BATSE channels, the photon flux peak in each
individual channel will inevitably be offset producing what we measure as lag.
We test this hypothesis by measuring the rate of $E_{pk}$ decay ($\Phi_{o}$)
for a sample of clean single peaked bursts with measured lag. We find a direct
correlation between the decay timescale and the spectral lag, demonstrating the
relationship between time delay of the low energy photons and the decay of
$E_{pk}$. This implies that the luminosity of a GRB is directly related to the
burst's rate of spectral evolution, which we believe begins to reveal the
underlying physics behind the lag-luminosity correlation. We discuss several
possible mechanisms that could cause the observed evolution and its connection
to the luminosity of the burst.
