Researchers have identified sources of individual differences in writing across beginning and developing writers. The aim of the present study was to further clarify the sources of this variability by investigating the extent to which there are differences in genetic and environmental factors underlying the associations between lexical diversity, syntactic knowledge, and semantic cohesion knowledge in relation to writing. Differences were examined across two developmental phases of writing: beginning (i.e., elementary school) and developing (i.e., middle school). Participants included 262 twin pairs (M[subscript age]= 10.88 years) in elementary school and 247 twin pairs (M[subscript age] =13.21 years) in middle school. Twins were drawn from the Florida Twin Project on Reading, Behavior, and Environment. Biometric models were conducted separately for subgroups defined by phase of writing development. Results indicated significant etiological differences in writing components across the two phases, such that effects associated with genes and non-shared environment were greater while effects associated with shared environment were lower in developing writers as compared to beginning writers. Furthermore, results showed that child-specific environment was the largest contributor to individual differences in writing components and their covariation for both beginning and developing writers. These results imply that even direct instruction about writing in schools may be having different effects on children based on their unique experiences.