ABSTRACT
Nearly all human characteristics and behaviors run in families, that is to say, relatives and people living together tend to resemble each other more than two individuals chosen at random from the same population. The core biological and social unit for studies of families consists of two parents, who have one or more offspring. They are thus biologically related due to transmission of shared chromosomes and genes from parents to their children. Parents and their children are defined as first-degree relatives, and through other biological relatives, more extensive biological relationships can be identified. For quantitative, continuous traits, familial correlations can be estimated for pairs of relatives. The intraclass correlation coefficient estimates the degree of resemblance between two family members of the same generation (such siblings or cousins), while interclass correlation estimates the degree of resemblance between two family members from different generations.
