The presentation of the voluminous results in Chapters 5–15 calls for discussion of their meaning and implications. Above all, there is need for the formulation of a theory to account for and explain them. In Chapter 16, a three-stratum theory of cognitive abilities is offered, with discussion of how the theory relates to current views of the nature of cognitive processes and performances. Moreover, the results themselves have implications for further research and for the resolution of many problems concerning cognitive abilities – their measurement and their applications in various domains of practical life. Chapter 17 touches on some of these issues. Chapter 18 is directed toward implications for further research on cognitive abilities, test construction, test use, and public policy on testing.

One could address a very long list of issues that pertain to cognitive abilities, their measurement, and their implications. Many of these issues are related, in a general way, to what is often called the “nature-nurture problem,” that is, the problem of whether there are genetic determinants of abilities, and of whether the environment, experience, learning, education, and training can contribute to the development and enhancement of abilities. There has been interest in how abilities mature or otherwise change with age – through childhood and adolescence to adulthood, and in the later stages of life. There is also the issue of whether abilities are malleable through specific interventions designed to improve them or alter their operation.