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addiction in humans". It has turned attention away from the 

 concept of an "addictive personality" to a realization that 

 addictive drugs share common chemical effects in the brain^* . 



Despite the wide chemical diversity among different 

 addictive substances, a property that most of them share is the 

 ability to affect the regulation of a chemical called dopamine in 

 parts of the brain that are important to emotion and 

 motivation". It is now believed that it is the effect of 

 addictive substances on dopamine that is responsible for driving 

 animals to self -administer these substances and for causing 

 humans to develop addictions". 



Regulation of dopamine, rewards the activity, and causes the 

 animal or person to repeat the activity that produced that reward 

 " ,^*. The process by which the regulation of dopamine leads an 

 animal or a human to repeat the behavior is known as 

 "reinforcement"." Drugs that have the ability to directly 

 modify dopamine levels can produce powerfully ingrained addictive 

 behavior^* . " 



One of the ways that researchers now test the addictive 

 properties of drugs is to determine whether animals will self- 

 administer that substance and then to determine whether the 

 animals will stop self -administering if the chemical action of 

 the substance is blocked by the simultaneous administration of 

 another drug that prevents the firit substance from acting in the 

 brain. Data gathered over the past 15 years have documented that 

 laboratory animals will voluntarily self -administer nicotine", 



