The proportion of individuals who have a phenotype at a specified point in time or within a defined timeframe (for example, 1 year).
DISABILITY ADJUSTED LIFE
YEAR
The years of life that are lost due to premature mortality or disability.
PSYCHOPATHOLOGY
A psychiatric disease, or the manifestation of a psychiatric disease.
Addictions are psychiatric disorders that are associated with maladaptive and destructive behaviours, and that have in common the persistent, compulsive and uncontrolled use of a drug or an activity. Addictive agents induce adaptive changes in brain function; these changes are the bases for tolerance and for the establishment of craving, withdrawal and AFFECTIVE DISTURBANCE, which persist long after consumption ceases 1 . This self-maintaining and progressive neurobiology of addictions makes them chronic and relapsing disorders.
The addictions are a worldwide public-health crisis, and exert corrosive effects at family and societal levels, leading even to the narco-political and narco-economic domination of countries and regions. The World Health Organization (WHO) estimated that there are 2 billion alcohol users, 1.3 billion tobacco users, and 185 million users of illicit drugs (see the WHO reports in the Online links box). In 2001, these three categories together contributed to 12.4% of deaths worldwide (see the WHO reports in the Online links box). According to the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC) 2 , the one-year POINT PREVALENCE of DSM-IV (the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders issued by the American Psychiatric Association) substance-use disorders (excluding nicotine) is 9.35% in the United States, representing 19.4 million adults ๏ฎBOX 1๏ฏ. As well as causing ~590,000 deaths per year in the United States, addictive drugs also cause injury or illness to almost 40 million individuals 3 . On a population basis, alcoholism alone subtracts an average of 4.2 DISABILITY ADJUSTED LIFE YEARS (DALYs) per person, tobacco subtracts 4.1 DALYs and illicit drugs subtract 0.8 DALYs. For comparison, AIDS subtracts 6.0 DALYs and type 1 diabetes subtracts 0.1 DALYs 4 . The addiction disease burden is unequally distributed across countries; drugs have a higher impact in the United States and Europe than in developing countries where life expectancies are shorter (see the WHO reports in the Online links box).
Environmental and genetic factors contribute to individual differences in vulnerability to initiating use of addictive agents and in vulnerability to the shift from use to addiction. Cross-sectional studies on large twin samples also indicate that a mixture of environmental and genetic influences are shared between diseases 5 and provide a link between the normal range of behavioural variation and PSYCHOPATHOLOGY 6,7 . However, because addictions are in theory entirely preventable by law or individual choice, it has been argued that addictions are a low priority for genomic research 4 . Substance use shapes patterns of abuse and dependence, and conversely, addictive drugs are largely consumed by