There have been many publications in the subject of birth order and personality. However, the influence of birth order on personality is still subject to much debate today. This experiment aimed to investigate the validity of Sulloway’s predicted scoring of firstborns and lastborns in Big Five personality factors. Participants were asked to complete an online survey that required them to complete an abridged Big Five personality test, both in regards to themselves as well as in regards to their most distant sibling. The results of each survey, self-assessed (n=59) and peer-assessed (n=59), were analysed. Findings suggest that Sulloway’s prediction is partially valid, and that birth order only influences personality when observing siblings from within the same family, but not when self-assessing.