How savvy we are in responding to social cues is often determined by prior social experiences. Many of our personality traits are also likely shaped more by experience than by our genes. But what kind of experiences shape our social cognition and behavioral characteristics the most? Using a fish species with unique social attributes, we test two competing theories (complex vs costly social interactions) for the development of personality and social cognition. We conducted rearing experiments with female swordtails for over a year (from birth to adulthood) and manipulated exposure to a range of different mating interactions. Our findings suggest that the complexity of the social landscape had a greater impact in shaping specific behaviors (aggression, anxiety, activity) more than the costliness of these interactions. Meanwhile, we are finding that costly social experiences shape cognitive performance the most, but the complexity of our social experiences influences how we approach social decisions. Our research enables us to identify the social factors that contribute to the development of adult behavior, cognitive abilities, and (ultimately) the forces that shape the circuitry of the vertebrate brain.