  Different models of social influence have explored the dynamics of social
contagion, imitation, and diffusion of different types of traits, opinions, and
conducts. However, few behavioral data indicating social influence dynamics
have been obtained from direct observation in `natural' social contexts. The
present research provides that kind of evidence in the case of the public
expression of political preferences in the city of Barcelona, where thousands
of citizens supporting the secession of Catalonia from Spain have placed a
Catalan flag in their balconies. We present two different studies. 1) In July
2013 we registered the number of flags in 26% of the the city. We find that
there is a large dispersion in the density of flags in districts with similar
density of pro-independence voters. However, we find that the density of flags
tends to be fostered in those electoral district where there is a clear
majority of pro-independence vote, while it is inhibited in the opposite cases.
2) During 17 days around Catalonia's 2013 National Holiday we observed the
position at balcony resolution of the flags displayed in the facades of 82
blocks. We compare the clustering of flags on the facades observed each day to
equivalent random distributions and find that successive hangings of flags are
not independent events but that a local influence mechanism is favoring their
clustering. We also find that except for the National Holiday day the density
of flags tends to be fostered in those facades where there is a clear majority
of pro-independence vote.
