Climate economics models often assume that middle-income countries’ per capita incomes will catch up with those of today's high-income countries, while low-income countries will lag behind. This choice underrates the least developed countries’ chance to escape poverty. The consequences in terms of the resulting policy advice are stark: assumed slow growth for the poorest countries means lower projected business-as-usual emissions and, as a consequence, much weaker emissions reduction goals. But what if low-income countries grow more quickly, as China and India have? In the absence of emission reduction policies, fast economic development would mean higher business-as-usual emissions in the future, and would therefore require more ambitious global emissions reductions policies today. This chapter reviews current practices in modeling income growth in integrated assessment models of climate and economy; provides an empirical illustration of the impact that more optimistic economic development expectations would have on emissions mitigation targets; discusses the kinds of policies necessary to adequately reduce emissions per dollar of economic output in a scenario of robust economic development for the poorest countries; and concludes with recommendations for integrated assessment modelers.