  We study the community structure of the multi-network of commodity-specific
trade relations among world countries over the 1992-2003 period. We compare
structures across commodities and time by means of the normalized mutual
information index (NMI). We also compare them with exogenous community
structures induced by geographical distances and regional trade agreements. We
find that commodity-specific community structures are very heterogeneous and
much more fragmented than that characterizing the aggregate ITN. This shows
that the aggregate properties of the ITN may result (and be very different)
from the aggregation of very diverse commodity-specific layers of the multi
network. We also show that commodity-specific community structures, especially
those related to the chemical sector, are becoming more and more similar to the
aggregate one. Finally, our findings suggest that geographical distance is much
more correlated with the observed community structure than RTAs. This result
strengthens previous findings from the empirical literature on trade.
