In our project i2istudy the “game currency-minutes” can be earned in two ways by acting as a “teacher” and by inviting friends. Wanting to earn extra money, we have added a third way by directly purchasing “minutes” for real money. The result is surprising, although if you think about it, it’s easy to explain.
As a result, the "F-growth" factor, which totally reflects these parameters, increased.
This situation resembles one study about the visitors of the bar, who drank significantly more beer when offered free spicy chips. You can draw your own conclusions. In my opinion the behavior of people is obvious, and thus predictable.