“That indicates that the brain vulnerabilities had a family origin, though somehow the siblings of addicts — either due to environmental factors or other differences in brain structure — were able to resist addiction.”

Somehow.

The only motivation of purposive human behavior is free will. If you fail to understand this, even in the face of obvious evidence, you just might be an academic.

Meanwhile, never doubt it: Making absurd, hysterical claims about human motivation is as addictive as smoking crack cocaine — but it pays better.