Siblings share similar genetics and environments, however, their behavior can be quite different. To determine if within-litter variance in neonatal–maternal interactions predict adult sibling behavioral variance, we observed mother–pup interactions during postnatal days 1–8 in four Sprague–Dawley rat litters and measured adult offspring behavioral responses to social and physical novelty. Our results indicate that pup and maternal behavior varied by at least twofold within each litter, and that specific pup behaviors within each litter (perioral contact) were associated with increased maternal licking. Furthermore, siblings that received more licks and made more perioral contact during postnatal days 1–8 had longer latencies to approach novel objects in adulthood than siblings that received less licking and made less perioral contact. This within-litter variance in postnatal mother and pup behavior and offspring adult behavior indicates that early social dynamics within families are an important area to examine to understand the development of sibling variance. © 2011 Wiley Periodicals, Inc. Dev Psychobiol 54:199-206, 2012.