Chapter 1 aims to correct popular misinformation and summarizes how intelligence is defined and measured for scientific research. Some of the validity data will surprise you. For example, childhood IQ scores predict adult mortality.

Considering the continuing interest of the scientific community and the public in the genetic bases of intelligence, in this chapter we highlight three facets of the numerous studies in this broad area: (1) the trajectory of studies that have sought to elucidate the etiology of intelligence; (2) the relevance of the selected phenotype; and (3) the consequence of the chosen genetic mechanism. The use of three main approaches to the study of the genetic bases of intelligence (the linkage studies focused on heritability of the phenotypic trait, hypothesis-driven candidate region and gene studies, and genome-wide association studies) has resulted in a list of about 150 genes apparently associated with intelligence, but the discrepancy between the heritability estimates obtained in quantitative versus molecular genetic studies persists. One explanation of this discrepancy relates to the heterogeneity of the phenotypes often used in quantitative versus molecular genetic studies. The other states that, in addition to the genetic mechanism sampled by genome-wide association studies (i.e., the common variance-based mechanism), there may be other genetic mechanisms that influence individual differences in intelligence, in particular, epigenetic mechanisms. Their consideration may become a course-changing innovation for understanding the genetic bases of intelligence and providing a window onto the diversity of human abilities.