  The majority of Swift gamma-ray bursts (GRBs) observed at z > 6 have prompt
durations of T90 < 30s, which, at first sight, is surprising given that
cosmological time-dilation means this corresponds to < 5s in their rest frames.
We have tested whether these high-redshift GRBs are consistent with being drawn
from the same population as those observed at low-redshift by comparing them to
an artificially red-shifted sample of 114 z < 4 bursts. This is accomplished
using two methods to produce realistic high-z simulations of light curves based
on the observed characteristics of the low-z sample. In Method 1 we use the
Swift/BAT data directly, taking the photons detected in the harder bands to
predict what would be seen in the softest energy band if the burst were seen at
higher-z. In Method 2 we fit the light curves with a model, and use that to
extrapolate the expected behaviour over the whole BAT energy range at any
redshift. Based on the results of Method 2, a K-S test of their durations finds
a ~1% probability that the high-z GRB sample is drawn from the same population
as the bright low-z sample. Although apparently marginally significant, we must
bear in mind that this test was partially a posteriori, since the rest-frame
short durations of several high-z bursts motivated the study in the first
instance.
