Jen Wright at Experimental Philosophy:
… I’m writing this post because I found something even more interesting…and puzzling. Leaving people’s actual looking behavior aside, I found a very powerful effect — consistent across all the vignettes — for which side of the screen the potential victim (the fat guy or the baby) was on. When the victims were on the right-side of the screen, people’s would and should judgements were significantly higher (i.e., they were more willing to, and thought more strongly that they should, kill the victim to save the others), than when they were on the left-side of the screen.
So, does anyone have any suggestions as to what might explain this finding?
My guess is that it’s related to the previous findings that people tend to place active people on the left of passive people in pictures (though it seems to vary across languages). The easiest interpretation is that it seems more moral to sacrifice passive people than active ones. That would also fit with the pattern I pointed out before in our moral intuitions, that moral concern is highly contingent on whether we can be rewarded or punished by the beneficiary of our ‘compassion’.
Meteuphoric 
In cultures that read from left to right, “There are five people on a track and one person on the other” will create a mental picture with five people on the left and one person on the right. The one-person track will thus be seen as more expendable.
Therefore we should expect “There are five people on a track and one person on the other, the trolley is heading toward the five people” to elicit more “Yup, switch tracks” responses than “There is one person on a track and five people on the other, the trolley is heading toward the five people”. We need to test this!
Also, we need to test all “active left” effects on right-to-left readers (monolingual Arabic or Hebrew speakers being the obvious choice), and illiterate people. There is certainly an “active first” effect (e.g., many more languages place subject before object than the reverse), but how it translates to “active left” can be hemispheric specialization, or cultural ideas of “flowing left to right”. Have we tested if “Alice asked Bob” makes Alice more likable than “Bob answered Alice”?
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It’s also common vocabulary in film that protagonists are placed on the left and antagonists on the right side of the screen.
My speculation: people who read left to right are likely to look at the left hand image first, and there is some kind of endowment/status quo effect which places a higher valuation on the first image viewed.
I suspect this is one of the things the experiment is testing, since it is using eye tracking technology, and have made a comment on the original post asking about this.
Leopold, that’s a very interesting suggestion regarding languages. Someone really should test that.