For a considerable time, it has been suspected that long-only index funds are responsible for price increases on agricultural futures markets, particularly for grain. Utilizing market diagrams we analyze the long-term market impacts of long-only index funds. Our analysis reveals that long-only index funds by no means lead to surges in the market, but stabilize it. The market entry of long-only index funds lowers risk premiums, so farmers can hedge their price risk at lower costs. This favors storage and dampens seasonal price fluctuations at spot markets. Thus, the market entry of long-only index funds also favors consumers. Only longonly index funds themselves do not profit from the entry of additional long-only index funds. Indeed, their profitability is negatively correlated with the total number of long-only index funds on the futures market. Therefore, one should abstain from regulating the business of long-only index funds.