You could apply logic as well. For instance, assume:
a) a good economy is attractive,
b) a good economy results from policy decisions,
c) a country is the sum of its people, and
d) each citizen carries a (varying) fractional part of their native country's policy/history (the converse of c).
We can conclude that to maintain your good economy, you need to prevent countries merging with yours; i.e. you need to a good ingress filter.