  We embed the behaviour of tax evasion into the standard two-dimensional Ising
model. In the presence of an external magnetic field, the Ising model is able
to generate the empirically observed effect of tax morale, i.e. the phenomenon
that in some countries tax evasion is either rather high or low. The external
magnetic field captures the agents' trust in governmental institutions. We also
find that tax authorities may curb tax evasion via appropriate enforcement
mechanisms. Our results are robust for the Barab\'asi-Albert and
Voronoi-Delaunay networks.
