Barberis and Shleifer present a model where investors allocate funds based on the relative performance of investment styles. This model generates a rich set of predictions. “They show that style investing generates excess (too little) co-movement of assets within (across) styles than warranted by fundamentals, implying that reclassification of assets into a new style raises co-movement with respect to that style.” When a stock is added to the S&P 500 index, its co-movement with the index increases while its co-movement with stocks outside of the index declines.