Mutual fund investors are increasingly sticking to the simplicity of index funds. Data from the Association of Mutual Funds in India (AMFI) show the number of folios in such funds has more than doubled from 8.1 lakh to 19.1 lakh, over the last year. Assets under management (AUM) grew from ₹15,000 crore to ₹40,000 crore, a part of it is due to the market rally.