Subliminal: How Your Unconscious Mind Rules Your Behaviour. By Leonard Mlodinow. Pantheon; 260 pages; $25.95. Allen Lane; £20. Buy from Amazon.com, Amazon.co.uk

ASK someone to name a famous psychologist, and chances are they will pick Sigmund Freud (pictured), the bearded Austrian academic who came up with the idea of psychoanalysis. His ideas about the unconscious—a sort of shadowy basement of the mind that is inaccessible to rational thought, but which nevertheless influences people's behaviour—are part of popular folklore.

Although it remained popular at dinner parties, the idea of the unconscious fell out of favour among 20th-century psychologists, thanks to the rise of more scientific approaches to psychology. These focused purely on studying behaviour and refrained from theorising about the inner workings of the mind.

In his latest book, “Subliminal”, Leonard Mlodinow, a theoretical physicist who has been developing a nice sideline in popular science writing, shows how the idea of the unconscious has become respectable again over the past couple of decades. This development has been helped by rigorous experimental evidence of the effects of the subconscious and, especially, by real-time brain-scanning technology that allows researchers to examine what is going on in their subjects' heads.

That experimental evidence suggests that, as Freud suspected, conscious reasoning makes up a comparatively small part of the activity in our brains, with most of the work taking place where we can't tap into it. However, unlike Freud's unconscious (a hot, claustrophobic place full of repressed memories and inappropriate sexual fantasies about one's parents) the modern unconscious is a place of super-fast data processing, useful survival mechanisms and rules of thumb about the world that have been honed by millions of years of evolution.

It is the unconscious, for instance, that stitches together data on colour, shape, movement and perspective to create the sight enjoyed by the conscious part of the mind. Experiments on people with certain specific forms of brain damage, which remove the ability to perform some of these tasks, can reveal something about what is going on underneath. People with “blindsight” can respond to some visual stimuli even when they are not conscious of being able to see. Asked to walk down an obstacle-strewn corridor, they will dodge and weave and arrive at their destination unharmed because some residual data is still making its way into their brains—although at a level that is beneath the notice of their conscious minds.

The modern view of the unconscious mind may be more benign than Freud's, but it can still generate unwelcome impulses. Psychologists theorise that the well-documented tendency of humans to categorise almost every piece of information they come across is a survival mechanism that evolved to aid quick decision making. Yet it may also lie behind the tendency for human beings to group people into races, genders, creeds and the like, and then to apply certain characteristics—unjustifiably—to every member of that group.

The insights offered by modern science into the workings of the human mind are fascinating in their own right. But they also suggest that plenty of conventional wisdom about how humans behave may need rethinking. Mr Mlodinow notes that economic models, for instance, are built “on the assumption that people make decisions…by consciously weighing the relevant factors”, whereas the psychological research suggests that, most of the time, they do no such thing. Instead, they act on the basis of simple, unconscious rules that can sometimes produce completely irrational results. Mr Mlodinow's chapters on courts and the law are disturbing, in particular on how unreliable eyewitness evidence can be. This has been widely documented elsewhere. But there is good news in the book, as well: people informed of the biases and pitfalls of their unconscious brains are better at using their conscious minds to overrule them.