Cognition changes with age, and the amount and trajectory of change varies across individuals and functions. In this review, we argue that three general principles characterize adult life-span changes in brain and cognition. (1) Dimensionality: Many features of brain and cognition in aging and neurodegenerative disease represent quantitative differences along a continuum and are not unique to pathology. (2) Early influences – developmental origins of health and disease: Genetic dispositions and early environmental factors, likely even from fetal life, can have lasting impact on the brain and cognition. (3) Influences from a multitude of environmental factors: Current brain state and cognitive function will be determined by a combination of early factors and later environmental influences, often in interaction. These principles entail a model of age-associated cognitive decline and dementia based on dimensions rather than categories, life span rather than aging, and multidimensional systems-vulnerability rather than one major “biomarker.”

This chapter examines the reciprocal relation between intelligence and achievement, particularly within academic domains such as verbal ability and mathematical ability. In particular, the chapter examines the specific knowledge needed for successful performance on tests of verbal ability that focus on decoding or reading comprehension, and tests of mathematical ability that focus on solving arithmetic computation problems or arithmetic word problems.

Massive IQ gains over time showed that obsolete norms had inflated estimates of the effects of intervention, adoption, and aging; and misdiagnosis of whether individuals had met IQ cutting lines that affected everything from the administration of the death penalty to who should benefit from special education. There were also important studies cited in the literature as if they could be taken at face value – the adoption study by Skodak and Skeels, for example. In America, obsolete norms had turned the death penalty into a lottery: you survive if you took a current test and got sixty-eight; you die if you took an obsolete test and got seventy-five. Research on the causes of IQ gains showed that environmental factors had a potency hitherto unappreciated, illuminated the history of cognitive progress in the twentieth century and its social significance, and recast the debate about group differences in IQ.

This chapter poses some of the principal questions that will confront the future of intelligence research. Among these questions are (1) What is the role of the brain in intelligence and of intelligence-enhancing drugs upon the brain? (2) Does culture affect what intelligence is or just what it is conceived of as being? (3) What are the genetic bases of intelligence? (4) Can intelligence of individuals be meaningfully increased by environmental interventions? (5) Is high intelligence having negative as well as positive effects on the world, and what is to be done about it? (6) Will artificial intelligence ultimately be dangerous to the world? (7) What effects are social media and the Internet more generally having on both individual and collective intelligence? (8) Why are people who are so intelligent at times so lacking in rational and wise thinking? (9) Are there noncognitive (e.g., emotional, attitudinal, motivational) aspects of intelligence and, if so, what are they? (10) What kinds of environmental factors inadvertently decrease intelligence?

This chapter reviews the intellectual history of the study of intelligence. It provides an overview of the field today. Then it reviews the early history of the field, dating back to Sir Francis Galton and then Albert Binet, Charles Spearman, and their successors. It discusses the theory of general intelligence and competitors to the theory such as the structure-of-intellect model proposed by J. P. Guilford. The chapter also describes in some detail various approaches to testing intelligence. It further describes the work of Jean Piaget, as well as biological and cultural views on intelligence. It then describes the theory of multiple intelligences and the theory of successful intelligence. It further describes the Flynn effect. Then it draws conclusions.