The Social Brain Hypothesis regards large social groups of primates and particularly in humans as the result of the development of cognitive skills necessary for social interactions. However, it has not yet been discovered how humans can maintain numerous social relationships at the same time, and why differences exist between the size and nature of peoples social networks. This study tested whether cognitive capacity, measured by general intelligence, accounts for differences in social network size among humans, or if other factors such as personality, altruism, empathy, systemizing and gender play a more important role in human social networks than previously assumed. A novel way of measuring developing social networks in an undergraduate population was successfully tested and a reliable questionnaire assessing social network size was designed. Results showed that extraversion and comprehension IQ positively affected social network size, and that, overall, female participants had larger social networks. Extraversion proved to be the only factor affecting the two most intimate circles of an individual’s social network, the support clique and the sympathy group. This study has implications for future research as it showed that additional factors like personality affect human social networks and should therefore be included in existing cognitive models.