This study demonstrates how self-evaluation processes explain subgroup differences in ratings of life satisfaction (population heterogeneity). Life domains differ with regard to the constraints they impose on beliefs in internal control. We hypothesized that these differences are linked with cognitive biases in ratings of life satisfaction. In fact, two subgroups of respondents needed to be distinguished, for which life satisfaction scores were non-equivalent measures. Self-evaluation processes also helped to explain age-related differences in life satisfaction. Age was unrelated or positively related to life satisfaction in a subgroup of respondents who perceived comparatively high levels of control over high-constraining life domains such as work, income, and standard of living. However, age yielded a substantial negative relationship with life satisfaction among participants who reported reduced levels of control in these domains. Results from a German representative sample were replicated with data from an online survey.