We study whether stock indexes can be a mechanism for transforming long-standing corporate behavior. After decades of low corporate profitability in Japan, the JPX-Nikkei400 Index was introduced in 2014. Each year the index selected 400 large and liquid firms deemed best performing in terms of profitability; membership is considered highly prestigious. We document that index-inclusion incentives have led firms to increase return on equity proportionally by 41% on average, via higher margins, efficiency, or shareholder payouts, depending on where they had slack. These incentives are driven by the prestige associated with the index, rather than direct-pecuniary or capital-market benefits. Back-of-the-envelope estimates suggest that the index-inclusion incentives accounted for 16% of the average increase in aggregate annual earnings and 20% of the growth in aggregate market capitalization over our sample period. Stock indexes can affect behavior by functioning as a source of prestige.