RESUMEN
The development of scientific thinking was assessed in 1,581 second, third, and fourth graders (8-, 9-, 10-year-olds) based on a conceptual model that posits developmental progression from naïve to more advanced conceptions. Using a 66-item scale, five components of scientific thinking were addressed, including experimental design, data interpretation, and understanding the nature of science. Unidimensional and multidimensional item response theory analyses supported the instrument's reliability and validity and suggested that the multiple components of scientific thinking form a unitary construct, independent of verbal or reasoning skills. A partial credit model gave evidence for a hierarchical developmental progression. Across each grade transition, advanced conceptions increased while naïve conceptions decreased. Independent effects of intelligence, schooling, and parental education on scientific thinking are discussed.