This study assessed how many motivational factors were required to explain scores for prosocial behavior, as measured by the Spanish version of the prosocial tendencies measure. A sample of 472 middle class children and adolescents, both sexes, from Buenos Aires, Argentina, completed the prosocial tendencies measure. This instrument presents prosocial behavior in six types: altruistic, compliant, emotional, public, anonymous, and dire. However, there is evidence that there should be a valid four-factor solution. To verify which factor structure better fit the empirical data obtained, two confirmatory analyses were performed. The results suggest that a four-factor structure (altruistic, public, anonymous, and responsive) is a more parsimonious explanation of the prosocial responses, compared to a six-factor solution. Finally the correlations between the four dimensions reinforced the hypothesis that altruism is the only prosocial behaviour that is selflessly motivated.