Dr Lawrence Whalley of the University of Aberdeen, who led the research, said the explanation could be that smoking causes oxidative stress - cumulative damage caused by molecules called "free radicals" - to organs including the brain.

BBC: NEWS | Health | Smoking 'is bad for your brain'

That's not the case now, because the economic damage facing the US is cumulative.

BBC: Who, What, Why: Who first called it a 'fiscal cliff'?