|This paper modifies and extends Galor and Zeira (1993, REStud) influenced model by adding the heterogeneity in abilities, rather than the heterogeneity in intergenerational bequest transfers due to the difference in initial endowment, to analyze the effects of international market openings on the evolution of middle-income class. The author argues that the timing of international trade openings matters for this evolution. If the economy engages to international markets too early, particularly when it is not developed enough, then the international trade activities create adverse effects on education investment among middle-income class, making this class shrink. In contrary, if the economy is sufficiently developed then international markets opening would be beneficial for human capital formation of the middle-income class. These movements, under the international markets opening, specifically hinge on the level of intergenerational bequest transfers which are determined by the level of development. Overall, the topic is interesting and important in understanding the dynamics of middle-income class. Given the current version of the paper, however, the prize committee strongly encourage the author to focus more on clarifying the mechanism behind the theoretical results as well as to discuss further about alternative directions of effects of international trade, which depend on the comparative advantages of the economy, on the income distribution. These to-do-works promise to strengthen the scientific contributions of the paper.