While researchers have identified many ecological conditions that impact life chances, there are many yet to be identified. This study identifies the size of the middle-income population as an ecological condition that is related to life outcomes. Specifically, this study explores the relationship between the size of the middle-income population and economic mobility, educational outcomes, and healthy habits. I use a social networks and social capital theoretical orientation to provide a potential explanation of the ecological influence of the middle-income population. Larger middle-income populations might increase ecological network connectivity, leading to greater ecological access to social capital that can improve life chances. Using commuting zone level data from Opportunity Insights, I find that a larger middle-income population is significantly related to higher upward mobility rates, higher test scores, and lower high school dropout rates. Robustness testing reveals a sensitive relationship between the size of the middle-income population and low-income smoking habits and low-income exercise habits. The findings suggest that the size of the middle-income population is an important ecological characteristic to consider for understanding social stratification.