The belief that a parent favors one child, has a significant impact on the relationship between children and often is the trigger of the antagonism between brothers and sisters. It is less important whether the child’s belief is true or not than the fact that its negative effects can be experienced by the whole family. The aim of this research was to determine whether there is a connection between birth order and perception of interpersonal parental investment. The sample was comprised of 155 respondents aged 18 to 33 years. Among the respondents were 38.1% firstborns, 40% middleborns and 23.9% lastborns. In the survey was used a Questionnaire of quality of relationship between siblings in adulthood. Participants on the position of the firstborns and lastborns in their families believed that they are the ones who get the most of interpersonal parental investment, particularly affection and closeness of mother and father’s support. Middleborns believe that parents invest equally in all children in the family. Also, all respondents believed that other brothers and sisters agree with their perceptions of interpersonal parental investment. Research showed that there is a connection between birth order and perceptions of parental investment and this could be a starting point for a future research aimed at determining whether the relationship between brothers and sisters is a possible consequence of the perceived or actual parental investment.