  We present HST/WFC3 narrowband imaging of the H-alpha emission in a sample of
eight gravitationally-lensed galaxies at z = 1 - 1.5. The magnification caused
by the foreground clusters enables us to obtain a median source plane spatial
resolution of 360pc, as well as providing magnifications in flux ranging from
~10x to ~50x. This enables us to identify resolved star-forming HII regions at
this epoch and therefore study their H-alpha luminosity distributions for
comparisons with equivalent samples at z ~ 2 and in the local Universe. We find
evolution in the both luminosity and surface brightness of HII regions with
redshift. The distribution of clump properties can be quantified with an HII
region luminosity function, which can be fit by a power law with an exponential
break at some cut-off, and we find that the cut-off evolves with redshift. We
therefore conclude that `clumpy' galaxies are seen at high redshift because of
the evolution of the cut-off mass; the galaxies themselves follow similar
scaling relations to those at z = 0, but their HII regions are larger and
brighter and thus appear as clumps which dominate the morphology of the galaxy.
A simple theoretical argument based on gas collapsing on scales of the Jeans
mass in a marginally unstable disk shows that the clumpy morphologies of high-z
galaxies are driven by the competing effects of higher gas fractions causing
perturbations on larger scales, partially compensated by higher epicyclic
frequencies which stabilise the disk.
