In the modern world, many Buddhists act like they don't really believe in kamma, by which skepticism they endanger their own future. However, there is a parallel to cetana (volition), in Western psychological research which provides some additional support for the validity of the idea of kamma.
First, I will present two quotations from Bhadantacariya Buddhaghosa, Bhikku Syanamoli, tr., VISUDDHIMAGGA, 1964, Colombo: A. Semage.
XVII 251 "firstly, kamma-process becoming in brief is both volition [cetana] also and the states of consciousness, etc. associated with volition."
XVII 252 "thirteen kinds of volition"
In Western psychological research, volition is called conation.
Now, the various psychological processes, when they occur, give rise to memories of the processes. Therefore, when one acts, the conation part of the behavior gives rise to conation memories.
These conation memories have the special property of causing behaviors to happen, because conation itself has the property of causing behaviors to happen.
Now, association of one process/event, perceptual or behavioral, with others is an almost universal process. In the case of the conation we are considering, the false self of oneself and the false self of the recipient of a behavior we do, are associated with the conation, and therefore the memories of them are associated with the conation memory.
Associations tend to generalize when conditions are right for that to happen. That is to say other processes/events which are similar become more like the original process/event.
Now what will happen when a conation memory returns to an active state, often in a future lifetime?
The conation memory will cause a process/event very similar to the process/even involved with the original conation.
However, the other false self the original behavior was directed towards will usually no longer be present. So generalization will cause one's own false self to to replace the other false self in the process/event repetition. Thus, in various ways, what one does to others will be done to oneself in the future whenever the conation memory becomes active.
That gives us strong additional evidence that the idea of kamma is in fact true.
Of course, other content of the behaviors/events will be part of what happens, along with the conation/cetana.
I think the quotation about the 13 kinds of cetana is important for future research on conation. There should be additional knowledge about kamma that can be obtained by research on those thirteen kinds.