The Social Animal: The Hidden Sources of Love, Character and Achievement. By David Brooks. Random House; 448 pages; $27. Buy from Amazon.com

FORTY years ago Walter Mischel, an American psychologist, conducted a famous experiment. He left a series of four-year-olds alone in a room with a marshmallow on the table. He told them that they could eat the marshmallow at once, or wait until he came back and get two marshmallows. Recreations of the experiment on YouTube show what happens next. Some eat the marshmallow immediately. Others try all kinds of strategies to leave the tempting treat alone.

Nothing surprising there. The astonishing part was the way that the four-year-olds' ability to defer gratification was reflected over time in their lives. Those who waited longest scored higher in academic tests at school, were much less likely to drop out of university and earned substantially higher incomes than those who gobbled up the sweet straight away. Those who could not wait at all were far more likely, in later life, to have problems with drugs or alcohol.

In his fascinating study of the unconscious mind and its impact on our lives, David Brooks uses this story to illustrate how the conscious mind learns to subdue the unconscious. This is not a question of iron will, but about developing habits and strategies that trigger helpful processes in the unconscious, rather than unproductive ones. What matters is to learn to perceive property, people or situations in ways that reduce the temptation to lie, to steal or behave in a self-destructive way.

In building his argument, Mr Brooks, a columnist on the New York Times, uses as his framework the lives of two imaginary characters, Harold and Erica, whom he follows from cradle to grave. This deliberate homage to Jean-Jacques Rousseau's 1762 work, “Emile: or On Education”, is not a success. Rousseau's book was banned and burned by angry readers. Harold and Erica are not that interesting, and they do get in the way of Mr Brooks's otherwise intriguing argument.

The author's aim is to show how recent research has illuminated the complex processes of the brain. “We have inherited an obsolete, shallow model of human nature,” he argues. Study after study, many of them little known, show that people take decisions about their jobs, relationships, actions and morals in ways that involve a complex interaction between the conscious and the unconscious mind. The most important decisions begin in the realm of the unconscious, although they are often influenced by the conscious.

The shaping of this delicate balance begins early in life: the children who were best at leaving their marshmallow on the plate tended to come from stable, organised homes. Culture and the community in which a child is raised help to build the way the conscious and unconscious intertwine. Mr Brooks recounts a survey of diplomats who failed to pay parking fines in New York. By far the worst non-payers came from countries where corruption is endemic: Egypt, Pakistan, Nigeria and so on. By contrast, diplomats from Sweden, Denmark, Japan, Israel, Norway and Canada had no unpaid fines at all. “Thousands of miles away from home,” Mr Brooks writes, “diplomats still carried their domestic cultural norms inside their heads.”

What does all this mean for public policy? Mr Brooks complains that policies too frequently rely on an overly simplistic, rationalist view of human nature. That may be true, but all too many daft policies rely on the collective reluctance of the voters to leave marshmallows uneaten on the table. More to the point, how can a country curb crime, create true equality and reduce the social and economic costs of bad decisions? Education systems exist mainly to build the rational mind, and yet the decisions that are most important in making people happy are the ones in which reason plays little or no part: the development of friendships and the choice of a spouse. Public policy has largely ignored this.

So Mr Brooks has done well to draw such vivid attention to the wide implications of the accumulated research on the mind and the triggers of human behaviour. But more books, preferably without Harold and Erica, remain to be written about the way societies should respond to what we now know.