Children's negative emotionality (NE) is frequently associated with parental negativity, but causal understanding of this relationship is limited. In addition, little is known about how genetic and environmental factors affect this relationship during middle childhood. We addressed these gaps by applying a quantitative genetic analysis to cross-lagged associations between mothers' and fathers' parental negativity and children's NE during middle childhood. The sample comprised of 456 families when the children were 6.5 years old, and 401 families when the children were 8/9 years old. Mothers' and fathers' negativity and children's NE were assessed using questionnaires. Results showed that variation in parental negativity was mainly accounted for by the environment shared by children, with some indication of an evocative effect of the children's genes on mothers, but not fathers. Children's NE was accounted for by both genetic and shared environmental influences. Parental negativity and children's NE had moderate continuity over the course of two years. Mothers' (but not fathers') negativity when the children were 6.5 years old predicted change in children's NE (rated by the same or the other parent) toward age 8/9 years, but not the other way around. Shared environmental influences were the main contributor to the association between earlier mothers' negativity and later children's NE. Thus, although children's NE was partially heritable, and parenting too was partially accounted for by children's genes, the association between parental negativity and children's NE, at this age, reflects environmental effects and is compatible with mothers' influence on children.