Research methods in both behavioral genetics and personality are currently at a crossroads. This chapter examines the disagreement about the genetics of behavior by reformulating its methodological foundation of twin and family studies. It applies the reformulation of older methods to gain realistic understanding of the newer ones that capitalize on the availability of measured DNA. The chapter highlights a particularly problematic aspect of scientific inference in the human behavioral sciences: the inference of causality from nonexperimental data. Religiosity was measured using four items (rated on four-point or five-point ordinal scale) assessing importance of religion, frequency of prayer, attendance at religious services, and attendance at youth groups. Random effects model was estimated in monozygotic (MZ) twin pairs using PROC MIXED in SAS. Linkage analysis has been the earliest molecular method to be adopted in the study of behavior because it requires minimal knowledge of actual genetic sequence.

This chapter describes the techniques that facilitate new understanding of neuroimaging results by adding a molecular genetic approach to examining neuroimaging phenotypes. It provides Mendelian genetics to highlight the point that variation in a single gene can affect many of the phenotypes measured in neuroimaging. The chapter discusses the expansion of trinucleotide repeat DNA sequences and imprinting, to the characteristics the inheritance and course of a disorder. The genetic analysis of pairs of siblings examines the degree of similarity between affected siblings for a quantitative phenotype. The transmission disequilibrium test (TDT) is based on the fact that a parent donates a single chromosome of each pair and, thus, one allele of any particular genetic marker. The population association method is commonly used but has a serious caveat, which is the association test between two groups of unrelated individuals.