Albeit used in the keywords of the paper, feather pecking is not an addiction, but normal behavior turned pathological in artificial (overcrowded) environments.
Addiction typically arises through overstimulation of reward centers in the brain, most notably the dopaminergic pathways involving the nucleus accumbens. Massive dopamine release, e.g. through cocaïne or meth intake, results in euphoria (Wise & Bozarth, 1985). When the drug's effects wear off it is followed by the 'crash', characterized by dysphoria and a longing for more drugs (CSAT, 1999). Gambling addiction works much in the same way.
Addiction develops because of an interplay of dependence and tolerance. Dependence is caused by downregulation of dopaminergic responses in the reward centers. Notably, dopamine receptors are downregulated when dopamine agonists like stimulant drugs of abuse are chronically administered. Downregulation of dopaminergic responses causes the physiology of the reward pathway to adapt to the presence of dopamine agonists. While the drug is in the system, everything is OK. However, once the levels of the dopamine agonist wears off, the body enters a state of lack of dopaminergic responses, causing dysphoria and depression. Tolerance develops as the liver and other tissues start breaking down the drug more efficiently due to upregulation of the necessary catabolic enzymes. Hence, more of the drug is needed, and the user becomes more and more tolerant. The interplay of dependence and tolerance form the dreaded vicious, downward spiral of addiction.
The reward centers have been crucial in evolution, as they provide motivation to vital acts of life, including eating and sex (University of Colorado, Boulder). Without a proper reward system, why eat or reproduce? However, drugs and other addictive acts such as gambling also activate the reward system.
Highly purified drugs such as crystal meth and cocaïne result in massive dopamine release that is unparalleled by normal physiologic processes such as eating. Before the occurrence of stimulants and things like gambling, the reward system was nothing but essential to life and evolution. Now, things are different and man needs to adapt to these novel, artificial pleasures in life.
- Treatment Improvement Protocol (TIP) Series, No. 33. Center for Substance Abuse Treatment. Rockville (MD); 1999
- Wise & Bozarth, Psychiatr Med (1985); 3(4): 445-60