Previous research has revealed the importance of body mass for biological fitness. However, studies that explore body mass index (BMI) in life-history perspective are still scarce. In the present research, we tested the hypothesis that a harsh environment facilitates faster growth, thus resulting in higher BMI; consequently, higher BMI should be related to fitness-related advantages, especially in individuals who grew up in a harsh environment. The research is conducted on a community sample of young adults (N = 218). In addition to BMI, we measured economic poverty and family dysfunctions as indicators of environmental harshness. A composite measure of short-term mating and the early onset of sexual activity was used as an operationalization of mating success, representing a fast life-history behavioral pattern. The results showed that BMI was positively related to environmental harshness and mating success; furthermore, a harsh environment and mating success were also positively associated. Finally, an interaction between BMI and poverty was detected: Mating success was especially highly pronounced in individuals with elevated BMI who grew up in conditions of economic scarceness. The results are in line with life-history theory, especially with developmental theories of psychosocial acceleration. Study findings suggest that higher BMI is a part of a fast life-history pathway, and it represents an adaptive morphological response to harsh environment. (PsycINFO Database Record (c) 2018 APA, all rights reserved)