But such warnings don’t work. Worldwide, people continue to inhale 5.7 trillion cigarettes annually  a figure that doesn’t even take into account duty-free or black-market cigarettes. According to World Bank projections, the number of smokers is expected to reach 1.6 billion by 2025, from the current 1.3 billion.

Image Credit... Henrik Drescher

A brain-imaging experiment I conducted in 2006 explains why antismoking scare tactics have been so futile. I examined people’s brain activity as they reacted to cigarette warning labels by using functional magnetic resonance imaging, a scanning technique that can show how much oxygen and glucose a particular area of the brain uses while it works, allowing us to observe which specific regions are active at any given time.

We tested 32 people (from Britain, China, Germany, Japan and the United States), some of whom were social smokers and some of whom were two-pack-a-day addicts. Most of these subjects reported that cigarette warning labels reduced their craving for a cigarette, but their brains told us a different story.

Each subject lay in the scanner for about an hour while we projected on a small screen a series of cigarette package labels from various countries  including statements like “smoking kills” and “smoking causes fatal lung cancers.” We found that the warnings prompted no blood flow to the amygdala, the part of the brain that registers alarm, or to the part of the cortex that would be involved in any effort to register disapproval.

To the contrary, the warning labels backfired: they stimulated the nucleus accumbens, sometimes called the “craving spot,” which lights up on f.M.R.I. whenever a person craves something, whether it’s alcohol, drugs, tobacco or gambling.