The authors use data from the National Survey of Families and Households to examine a range of sociability behaviors for adults who grew up with and without siblings. Compared with adults who grew up with siblings, adults who grew up without siblings have less frequent social activities with relatives, and the difference is greater among those who did not live with both parents growing up. Differences in engaging in certain social events between adults who grew up without and with siblings vary by age. Differences in participation in sports-, youth-, or school-related group activities for those who grew up without and with siblings vary by gender. Thus, there are some differences in adult sociability behaviors between those who grew up with and without siblings; however, the pattern of findings in this study suggests that these differences are not large or pervasive across a range of sociability behaviors and may grow smaller with age. (Contains 5 tables.)