mental determinants of these risk behaviors and their underlying motivational systems; and (3) to discuss how the measurement of intermediate phenotypes (recently termed endophenotypes), such as personality and temperament, can advance our knowledge of the role of gene-environment interactions in risk behaviors and health.
Although definitions of health risk behaviors vary across studies, there are some generally accepted definitions that will be presented for the purposes of this chapter. With regard to tobacco use, the behavioral definition of smoking used in most prevalence studies includes having smoked more than 100 cigarettes in one’s lifetime and smoking every day or most days (CDC, 2005). Increasingly, studies of the determinants of tobacco use, including genetic studies, are using more refined behavioral definitions to characterize trajectories of smoking initiation and progression, as well as phenotypes related to nicotine addiction and smoking persistence (Audrain-McGovern et al., 2004b).
The definition of obesity is more straightforward. The World Health Organization (WHO) defines overweight as having a body mass index (BMI) from 25 to 30, and obesity as a BMI greater than 30 (WHO, 1998). Broadly speaking, physical activity includes any bodily muscular movements that produce energy expenditure (Caspersen et al., 1985; Pate et al., 1995). To reduce health risks, it is recommended that healthy adults engage in at least 150 minutes of moderate intensity physical activity per week (Pate et al., 1995), which can include brisk walking and some forms of aerobic exercise such as running and bicycle riding.
The importance of phenotype definition for investigations of genetic risk factors and gene-environment interaction cannot be overestimated.