Inclusion in a major index like the S&P MidCap 400 often leads to increased demand for a company's shares, as various mutual funds and ETFs that track the index may now be required to purchase the stock. This can lead to a short-term increase in stock price due to the heightened demand. For American Homes 4 Rent, this inclusion signifies recognition of its growth and stability, which can enhance its reputation among investors and potentially lead to a higher stock price in the short term.