Cosmological simulations of galaxies have typically produced too many stars at early times .
We study the global and morphological effects of radiation pressure ( RP ) in eight pairs of high-resolution cosmological galaxy formation simulations .
We find that the additional feedback suppresses star formation globally by a factor of \sim 2 .
Despite this reduction , the simulations still overproduce stars by a factor of \sim 2 with respect to the predictions provided by abundance matching methods for halos more massive than 5 \times 10 ^ { 11 } M _ { \odot } h ^ { -1 } ( ) .

We also study the morphological impact of radiation pressure on our simulations .
In simulations with RP the average number of low mass clumps falls dramatically .
Only clumps with stellar masses M _ { clump } / M _ { disk } \leq 5 \% are impacted by the inclusion of RP , and RP and no-RP clump counts above this range are comparable .
The inclusion of RP depresses the contrast ratios of clumps by factors of a few for clump masses less than 5 % of the disk masses .
For more massive clumps , the differences between and RP and no-RP simulations diminish .
We note however , that the simulations analysed have disk stellar masses below about 2 \times 10 ^ { 10 } M _ { \odot } h ^ { -1 } .

By creating mock Hubble Space Telescope observations we find that the number of clumps is slightly reduced in simulations with RP .
However , since massive clumps survive the inclusion of RP and are found in our mock observations , we do not find a disagreement between simulations of our clumpy galaxies and observations of clumpy galaxies .
We demonstrate that clumps found in any single gas , stellar , or mock observation image are not necessarily clumps found in another map , and that there are few clumps common to multiple maps .