The 14 learner-centered psychological principles developed by the American Psychological Association are categorized into cognitive/metacognitive factors, motivational/affective factors, developmental/social factors, and individual differences factors. The principles state that learning is most effective when the material is appropriately challenging, socially interactive, and relevant to the learner's interests and goals. Successful learners actively construct their own knowledge, use strategic thinking and self-reflection, and are intrinsically motivated to learn.