It is commonly said that childhood experiences shape adult behavior; that events that we may not even remember can have long-lasting or even permanent effects. In a new article by scientists at the Champalimaud Center for the Unknown in Portugal, published in Current Biology, experiments using zebrafish show that social experiences during the very first week of development impact behavior at an early larval stage, before the fish are considered social. This suggests that these experiences mark the fish much earlier than previously thought.