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(THE CONVERSATION) Why do they do it? This is a question that friends and families often ask of those who are addicted.

It’s difficult to explain how drug addiction develops over time. To many, it looks like the constant search for pleasure. But the pleasure derived from opioids like heroin or stimulants like cocaine declines with repeated use. What’s more, some addictive drugs, like nicotine, fail to produce any noticeable euphoria in regular users.

So what does explain the persis...