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. Such characteristics can be structural (height, bone structure, obesity) or physiological (muscle strength), behavioral (physical activity), or life-course related (such as number of children or lifespan). Correspondingly, abnormal conditions and disease often also run in families. Familial aggregation is thus a well-established observation, but the nature and causes of it depends on the trait in question. Familial aggregation is also well known for physical activity and exercise characteristics (2, 5, 21; see also Chapter 6). Given that there are multiple modes of physical activity and exercise, this chapter uses physical activity traits as the primary phenotypes in the examples described below.
