If there is an area of psychology that generates more heated debate than behavioral genetics it would be the field of intelligence research. While most of us acknowledge the differences among us in personality and even risk for mental illness, for some of us differences in intelligence seem more difficult to accept. I confess I am not completely sure why this is the case. Maybe it is because of the involvement of early intelligence researchers with the Eugenics Movement. Alternatively, maybe it is because the conclusions reached by some intelligence researchers seem to challenge our long-held beliefs about social equality, especially when those conclusions are biologically grounded. Regardless, intelligence, or as I prefer to call it general cognitive ability (GCA), has been a major focus of behavioral genetic research and we will use it as a prototype of behavioral genetic research on a quantitative psychological trait.As with the previous unit, we will begin with a brief discussion of what psychologists mean by intelligence or GCA. I will not try and review the vast empirical literature on the correlates of GCA; suffice to say that GCA is correlated, not always strongly, with many desirable outcomes including educational attainment, occupational achievement, health, mortality, criminal conviction, etc. Twin and adoption studies of GCA have implicated the importance of both genetic and nonshared as well as shared environmental influences. Behavioral genetic research has helped to identify features of the shared environment that appear to contribute to differences in intelligence, but, unlike with schizophrenia and other mental illnesses, it has been difficult to identify the specific genetic variants that contribute to the heritability of GCA. This unit will end with a discussion of genetic research on intellectual disability, an important application of intelligence research.Just FYI, one thing we will NOT consider in this unit but some of you will wonder about is developmental behavioral genetic research on intelligence. I promise that it will be covered in Unit 7.