The study tested how people choose index funds for investment. Subjects were given $10,000 to invest in S&P 500 index funds, with rewards based on the portfolio's return. The best choice was to invest all in the lowest-cost fund, but most subjects didn't do this. When fees were made clear, people shifted towards cheaper funds but still didn't minimize fees. Instead, they focused on past returns, even though this wasn't the best strategy. Those who picked high-cost funds were less confident in their choices.