PEOPLE are programmed for addiction. Their brains are designed so that actions vital for propagating their genes—such as eating and having sex—are highly rewarding. Those reward pathways can, however, be subverted by external chemicals (in other words, drugs) and by certain sorts of behaviour such as gambling.
In recent years, neuroscientists have begun to understand how these reward pathways work and, in particular, the role played by message-carrying molecules called neurotransmitters. These molecules, notably serotonin, dopamine and gamma-aminobutyric acid (GABA), hop between nerve cells, carrying signals as they go. Some drugs mimic their actions. Others enhance them. Either way, the body tends, as a result, to give up making them. At that point the person needs the drug as a substitute for the missing transmitter. In other words, he is an addict.
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