from Paul Graham | by Paul Graham

It's easier to see how fashion and convention (about e.g. how to smile) make everyone look the same if you turn pictures upside down. t.co/eqp5vsISNV

I think the reason is that we're so sensitive to differences between people's faces (since we need to be able to identify them) that this ordinarily dominates our perception of group photos. If we invert the photo, it's turned off, so we see the similarity.