Research problems and variables

Genetics of brain structure and intelligence

Genetic influences on brain morphology and IQ are well studied. A variety of sophisticated brain-mapping approaches relating genetic influences on brain structure and intelligence establishes a regional distribution for this relationship that is consistent with behavioral studies. We highlight those studies that illustrate the complex cortical patterns associated with measures of cognitive ability. A measure of cognitive ability, known as g, has been shown highly heritable across many studies. We argue that these genetic links are partly mediated by brain structure that is likewise under strong genetic control. Other factors, such as the environment, obviously play a role, but the predominant determinant appears to be genetic.

General design classifications

This column described the general design classifications of between-groups, within-subjects, and mixed designs. Remember that in between-groups designs, each participant is in only one group or condition. In within-subjects or repeated-measures designs, on the other hand, each participant receives all the conditions or levels of the independent variable. In mixed designs, there is at least one between-groups independent variable and at least one within-subjects independent variable. In classifying the design, do not consider the dependent variable(s). The classifications and descriptions presented in this column are for difference questions, using the randomized experimental, quasi-experimental, and comparative approaches to research. Appropriate classification and description of the design are crucial for choosing the appropriate inferential statistic, which is the topic of the next column and several to follow.