Dear Nina and friends,

thanks for everything, Nina. You are right about collective kamma, it
is commonly used in Mahayana schools, and a search on collective
karma will reveal that. I cannot for sure determine if the usage of
the word is more of a convenience, but I have the idea it appears in
Mahayana sutras.

I totally agree on the explanation you have given on the Theravada
perspective. And I apologise for my lack of knowledge and any
confusion due to that. Browsing the web, I have found what Sayadaw U
Silananda wrote on this topic:

One more thing that should not be applied to the doctrine of kamma is
the idea of mass kamma or collective kamma. There is no operation of
a collective kamma affecting a group of people. There may be, however
a group of people who do something together and who get the results
of their individual kamma together. In that case, the results of each
individual kamma is operating.

http://www.mahindarama.com/e-library/volition4.html

metta,
Yong Peng