  There are at least three different types of secular effects in the two-point
correlation functions in scalar quantum field theories in de Sitter space-time.
The first one is specific to de Sitter massless and tachyonic minimally coupled
scalar fields. The remaining two are generic and are encountered practically in
any non-stationary situation in quantum field theory. Furthermore there are
secular effects in the n-point correlation functions for low enough mass. They
are also specific to de Sitter quantum field theory. In this paper we focus on
the differences between the secular effects in two-point functions. We discuss
also their character in different patches of de Sitter space-time - global,
expanding and contracting Poincar\'{e} patches.
