ABSTRACT
As described in Chapter 2.1, obesity shows a familial correlation when analyzed as a continuous trait, such as body mass index (BMI¼weight/height2, kg/m2), and a familial aggregation when analyzed as a dichotomous trait (e.g., BMI 30 kg/m2). Such resemblance of traits in nuclear families may be due to identity of genes by descent or shared family environment. The investigation of the effects of these influences can be carried out in several different ways. Studies of twin pairs and studies of extended families are addressed in other chapters in Section 2 (see Chapters 2.1 and 2.3) (1). This chapter presents adoption studies, which are studies of families in which an individual has been removed from the biological parents early in life and reared in an adoptive family. In these studies, it is a fundamental requirement that the adoptive family is genetically unrelated to the biological relatives, but how adoptive families are selected varies between countries and over time in the same country. Adoptions may take place within other branches of the family, e.g., uncles and aunts, but these types of adoptions are obviously not suitable for assessing the separated effects of genes and shared environment.
