This paper examines whether the discount of diversified firms can actually be attributed to diversification itself, using recent econometric developments about causal inference with non-experimental data. The effect of diversification on firm value is unbiasedly estimated by matching diversified and single-segment firms on the propensity score??the predicted values from a probit model of a firm?s propensity to diversify. I apply this method on a sample of diversified firms that trade at a significant mean and median discount relative to single-segment firms of similar size and industry. I find that, when a more comparable benchmark based on the propensity score is used, the diversification discount as such disappears. Therefore, I find no reason to interpret the finding that diversified firms trade at a discount as evidence that diversification destroys value. In fact, my analysis of the propensity to diversify yields support to both value-creating and value-destroying arguments for diversification. I find that diversified firms trade at a significant industry-adjusted discount prior to diversification, which however does not seem to result from their future diversification. In addition, diversifying firms are present in industries with a lower q than those of their non-diversifying counterparts. I also find that, as predicted by agency theory, diversified firms prior to diversifying have a smaller percentage of their stock owned by institutions, insiders, and blockholders, a higher risk, and are likely to diversify into industries with a lower average leverage than their own. I find support for the resource-based theory of diversification as well in that firms are more likely to diversify when faced with opportunities for exploiting potential synergies and when they have enough financial resources to do so. As required for market power-based theories of diversification to hold, firms that diversify are present in industries with higher levels of concentration. More generally, certain industries appear to lend themselves more than others to either inward or outward diversification.