It is true that the conclusion that higher interest has not led to more lending is not based on rigorous data. It is rather based on the general observation that during the past year or so, loan funding volume has not noticeably increased at times when the highest interest rates offered at Zidisha increased.
The generalization that Zidisha lenders are not primarily attracted by financial returns is likewise not based on data, but rather a subjective assessment resulting from many years of survey responses, comments posted on our website and conversations with lenders.
(An important caveat is that many of our lenders do care about preserving the value of their lending funds over time, and often try to lend at interest rates that they believe will help them achieve this goal. But this is a very different motivation than profit maximization.)
I agree with you that offering dramatically higher interest may well attract more capital, from lenders who are motivated by profit rather than philanthropy. This would not necessarily be a bad thing, but it is not our mission or core area of expertise.
It will be interesting to see if a profit-based variation of Zidisha emerges, and whether it ultimately has a greater impact on the opportunities available to entrepreneurs in developing countries than our philanthropic platform.