The development of addiction requires the use of a substance and a subsequent chain of behavioral events that leads to addiction. The key steps in the development of addiction include the initiation of substance use, the conversion from experimental use to established use, and finally the development of addiction (see Figure 1). Each step is influenced by environmental and genetic factors, some of which are common to all steps, and others that are specific. For example, environmental factors, such as the availability of nicotine, alcohol, and drugs, play a role in each stage in the development of addiction, but accessibility of a substance is relatively more important in the initiation of substance use. Similarly, high cost of a substance through taxation can reduce initiation, use, and addiction; however taxation has a stronger influence on teenagers who have less money, thus limiting initial use. Family, twin, and adoption studies also convincingly demonstrate a substantial genetic contribution to the development of addiction to nicotine, alcohol, and illicit drugs. Heritability estimates for nicotine, alcohol, and drug addiction are in the range of 50%
