A family smoking index to capture genetic influence in smoking: rationale and two validation studies.

Despite a growing appreciation that genetic factors may impart vulnerability toward smoking behavior, only a modest consensus has been created about the specific genetic mechanisms that may underlie various aspects of smoking. A core feature of genetic contribution toward any complex human behavior is familial resemblance. Most previous attempts to index familial smoking have classified individuals into discrete categories, based on the number of smokers in a family. We discuss the development of a continuous measure of familial smoking, the Family Smoking Index (FSI), which is based on the proportion of smokers in first- and second-degree family members and provides a more precise weighting according to genetic proximity. We present the psychometric characteristics of the FSI as well as initial validation data from two studies. We also describe current and future directions for continued FSI validation and application.