Mechanistic explanations of personality functioning span a broad range, as is evidenced in the thoughtful commentary provided by Crowell et al. Mechanistic explanations of behavior and experience are complicated, however, by vague boundaries between potential causes and effects, across different levels of analysis as well as within the same level. This vagueness creates challenges in developing theories of how factors at lower levels of analysis are related to those at higher levels of analysis, as well as in disentangling causes and effects in general. Personality pathology is further complicated by relatively rare or even unique psychological and behavioral patterns of interest, and the long timescales over which development occurs. These challenges are important targets for future research on mechanistic explanations of personality.