ABSTRACT Project 4 Reading and math problems represent an important public health issue for children in that they are associated with various negative outcomes including school failure, limited occupational success, and juvenile delinquency (Geary et al., 2012; Reynolds et al., 2002). Of US fourth-grade students, one-fourth fail to reach even partial mastery of grade-level knowledge in reading, and one-fifth fail to reach partial mastery of grade-level knowledge in math (NCES, 2015), highlighting the prevalence of reading and math difficulties in childhood. Given we know that children who struggle in reading often also struggle in math, it is important to identify influences on the development of both reading and math. The overall goal of the proposed research is to uncover salient factors, including genetic and environmental influences, which contribute to the co- development of reading and math performance, at a critical developmental point (elementary school). We will identify the first nationally-representative US twin sample through the proposed National Project on Achievement in Twins (NatPAT). The NatPAT sample will comprise 7,668 pairs of twins located across the US, and will be uniquely situated to address the overall goal of the proposed research through four specific aims (SA). First, we will utilize a large national database of reading and math performance from schools across the country to ascertain the NatPAT twin sample. Using a cohort-sequential design starting in kindergarten, we will examine reading and math performance across elementary school in order to model genetic and environmental influences on reading and math (co-)development (Specific Aim 1). Second, we will model the genetic and environmental influences on the co-occurrence of reading and math difficulties, while also testing for sex differences (Specific Aim 2). Third, we will capitalize on publically available data to characterize the environmental contexts related to the (co-)development of reading and math performance (Specific Aim 3). Finally, we will capture important attitudinal individual differences dimensions and examine how they are associated with the (co-)development of reading and math performance (Specific Aim 4).