Yet Greenfield, 51, is the very antithesis of a fusty professor. She writes popular-science books, presents TV series like last year's Brain Story on the BBC and delivers public lectures. Last week she chaired a conference on music and the mind, which yielded intriguing insights into the way melodic patterns may be linked to the configuration of neuron networks in the brain. Her telegenic looks, designer clothes, accessibility and enthusiasm for her subject the mind and how it works have led the media to dub her Britain's only celebrity scientist. She doesn't mind the tag: it gives her a platform to speak out on issues about which she feels strongly, like the British government's announcement last week that marijuana will be downgraded to a low-risk drug whose use or possession is not an arrestable offense.
Greenfield thinks that moves to decriminalize the drug are based on "the greatest myth of all" the belief that cannabis is essentially harmless. "Having studied the effects of so-called soft drugs on the brain, I'm convinced that pot smokers are literally blowing their minds," she says. "We're starting to live in a drug culture, where instead of having fulfilled and interesting lives, people seek oblivion from the stresses of life through a chemical route." Despite her unhappiness with the government's actions, she finds the response from schoolchildren and young offenders, to whom she has taken her antidrug message, "hugely exhilarating. A 'Just Say No' approach would never work. But they are surprisingly receptive to hard facts."
The daughter of an electrician and a chorus dancer, Greenfield enrolled at Oxford to study philosophy and psychology after graduating from school with an unusual mix of math, Greek and Latin. "I was interested in why people behaved the way they did, what makes them fall in love or start wars. I discovered that science provided me with more answers than philosophy did," she explains. Her aim of "meshing science with society" has something of the zeal of the convert. "I'd like to see people attending scientific discussions the way they would go to a concert or the cinema," says Greenfield, who is married to fellow scientist and textbook author Peter Atkins. If anyone can persuade the public to devote their Saturday nights to science, she will.
TIME: How can marijuana "blow the mind"?
Greenfield : Drugs interfere with the careful interplay of chemical and electrical impulses between cells, throwing out the balance of the brain either by causing too many chemicals to be released or by stopping the cells' chemical transmitters reaching the vital receptors. The reason marijuana is so potent is that it has its own receptor in the brain. The more you smoke, the less sensitive the receptor becomes.
TIME: How is a person's behavior affected?
Greenfield : Research shows that the drug leads to impaired memory and coordination. These effects may be long-term and irreversible. There is strong evidence that marijuana can trigger schizophrenia. Then there's demotivational syndrome, an inability to focus on anything beyond the next fix.
TIME: Is it addictive?
Greenfield : That depends how we define addiction. Cannabis users have to take ever-larger quantities to achieve the desired effect. Studies show that about 10% of users can't stop their habit, despite wanting to do so.
TIME: Have you ever smoked marijuana?
Greenfield : For a student at Oxford in the '70s, it was difficult to avoid. So yes, I tried it, but like another famous student there, I didn't inhale!