  We examine a sample of 48 Swift/UVOT long Gamma-ray Burst light curves and
find a correlation between the logarithmic luminosity at 200s and average decay
rate determined from 200s onwards, with a Spearman rank coefficient of -0.58 at
a significance of 99.998% (4.2 sigma). We discuss the causes of the log L_200s
- alpha_>200s correlation, finding it to be an intrinsic property of long GRBs,
and not resulting from the selection criteria. We find two ways to produce the
correlation. One possibility is that there is some property of the central
engine, outflow or external medium that affects the rate of energy release so
that the bright afterglows release their energy more quickly and decay faster
than the fainter afterglows. Alternatively, the correlation may be produced by
variation of the observers viewing angle, with observers at large viewing
angles observing fainter and slower decaying light curves.
