THE idea that the human personality is a blank slate, to be written upon only by experience, prevailed for most of the second half of the 20th century. Over the past two decades, however, that notion has been undermined. Studies comparing identical with non-identical twins have helped to establish the heritability of many aspects of behaviour, and examination of DNA has uncovered some of the genes responsible. Recent work on both these fronts suggests that happiness is highly heritable.

As any human being knows, many factors govern whether people are happy or unhappy. External circumstances are important: employed people are happier than unemployed ones and better-off people than poor ones. Age has a role, too: the young and the old are happier than the middle-aged. But personality is the single biggest determinant: extroverts are happier than introverts, and confident people happier than anxious ones.

That personality, along with intelligence, is at least partly heritable is becoming increasingly clear; so, presumably, the tendency to be happy or miserable is, to some extent, passed on through DNA. To try to establish just what that extent is, a group of scientists from University College, London; Harvard Medical School; the University of California, San Diego; and the University of Zurich examined over 1,000 pairs of twins from a huge study on the health of American adolescents. In “Genes, Economics and Happiness”, a working paper from the University of Zurich's Institute for Empirical Research in Economics, they conclude that about a third of the variation in people's happiness is heritable. That is along the lines of, though a little lower than, previous estimates on the subject.

But while twin studies are useful for establishing the extent to which a characteristic is heritable, they do not finger the particular genes at work. One of the researchers, Jan-Emmanuel De Neve, of University College, London, and the London School of Economics, has tried to do just that, by picking a popular suspect—the gene that encodes the serotonin-transporter protein, a molecule that shuffles a brain messenger called serotonin through cell membranes—and examining how variants of that gene affect levels of happiness.

Serotonin is involved in mood regulation. Serotonin transporters are crucial to this job. The serotonin-transporter gene comes in two functional variants—long and short. The long one produces more transporter-protein molecules than the short one. People have two versions (known as alleles) of each gene, one from each parent. So some have two short alleles, some have two long ones, and the rest have one of each.

The adolescents in Dr De Neve's study were asked to grade themselves from very satisfied to very dissatisfied. Dr De Neve found that those with one long allele were 8% more likely than those with none to describe themselves as very satisfied; those with two long alleles were 17% more likely.

Which is interesting. Where the story could become controversial is when the ethnic origins of the volunteers are taken into account. All were Americans, but they were asked to classify themselves by race as well. On average, the Asian Americans in the sample had 0.69 long genes, the black Americans had 1.47 and the white Americans had 1.12.

That result sits comfortably with other studies showing that, on average, Asian countries report lower levels of happiness than their GDP per head would suggest. African countries, however, are all over the place, happinesswise. But that is not surprising, either. Africa is the most genetically diverse continent, because that is where humanity evolved (Asians, Europeans, Aboriginal Australians and Amerindians are all descended from a few adventurers who left Africa about 60,000 years ago). Black Americans, mostly the descendants of slaves carried away from a few places in west Africa, cannot possibly be representative of the whole continent.

That some populations have more of the long version of the serotonin-transporter gene has been noticed before, though the association has previously been made at a national, rather than a racial, level. In a paper in the Proceedings of the Royal Society, published in 2009, Joan Chiao and Katherine Blizinsky of Northwestern University, in Illinois, found a positive correlation between higher levels of the short version of the gene and mood disorders (China and Japan have lots of both) and with collectivist political systems. Their hypothesis is that cultures prone to anxiety tend towards systems that emphasise social harmony and away from ones that emphasise individuals' independence of each other.

This latter study may be a few steps too far along the road to genetic determinism for some people. But there is growing interest in the study of happiness, not just among geneticists but also among economists and policymakers dissatisfied with current ways of measuring humanity's achievements. Future work in this field will be read avidly in those circles.