  In this paper, we study the Target Set Selection problem from a parameterized
complexity perspective. Here for a given graph and a threshold for each vertex,
the task is to find a set of vertices (called a target set) which activates the
whole graph during the following iterative process. A vertex outside the active
set becomes active if the number of so far activated vertices in its
neighborhood is at least its threshold.
  We give two parameterized algorithms for a special case where each vertex has
the threshold set to the half of its neighbors (the so-called Majority Target
Set Selection problem) for parameterizations by the neighborhood diversity and
the twin cover number of the input graph.
  We complement these results from the negative side. We give a hardness proof
for the Majority Target Set Selection problem when parameterized by (a
restriction of) the modular-width - a natural generalization of both previous
structural parameters. We also show the Target Set Selection problem
parameterized by the neighborhood diversity or by the twin cover number is
W[1]-hard when there is no restriction on the thresholds.
