  Using a general network model with multiple activities, we analyse a
planner's welfare maximising interventions taking into account within-activity
network spillovers and cross-activity interdependence. We show that the
direction of the optimal intervention, under sufficiently large budgets,
critically depends on the spectral properties of two matrices: the first matrix
depicts the social connections among agents, while the second one quantifies
the strategic interdependence among different activities. In particular, the
first principal component of the interdependence matrix determines budget
resource allocation across different activities, while the first (last)
principal component of the network matrix shapes the resource allocation across
different agents when network effects are strategic complements (substitutes).
We explore some comparative statics analysis with respective to model
primitives and discuss several applications and extensions.
