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Back when Britain was brainy: applicants for the Mensa exam gather at the Russell Hotel in 1961
Average IQ is falling in Britain and beyond, explains Philip Hunter

Intelligence quotients (IQs) have risen in developed nations for almost a century. This phenomenon, named the "Flynn effect" after the New Zealand intelligence researcher James Flynn, was first identified in 1984 in the United States. It has been found to occur in all developed nations, and some others as well. The received wisdom became: IQs always go up. But this trend seems to be stopping and even reversing in some countries, research in Britain, Denmark and Norway has shown. It is discomfiting to find intelligence in decline. There is a strong association between a nation's IQ, its prosperity and health.

IQ testing is contentious and regarded by some as a crude indicator of ability or potential. When comparing nations, measured average IQ tends to be affected by class, nutrition, and cultural factors including education. There is also disagreement over the influences of nature and nurture.

IQs are collated such that the average test score is 100. The standard deviation - a statistical measure of the variation within a group - is set at 15. This means that if you score 115, you are one standard deviation from the mean; 130, two standard deviations, and so on. The Flynn effect is determined by asking subjects to sit old IQ tests. The finding has been that the average score of groups who take older tests is more than 100, a disparity that suggests rising average intelligence. The rate of increase varies between nations, but on average, IQ scores have risen by three points per decade.

But in Britain, research has found a reversal of this trend. This has also been the case in other nations, including Norway and Denmark (research is not extensive, but the implication is that declines may be occurring more widely). A study in 2009 led by James Flynn himself and published in Economics & Human Biology compared IQ scores obtained by British teenagers in 1980 and 2008, using the same test. The average had declined by two points on average, but by as much as six points among teenagers in the top half of the IQ scale, a fall that wiped out the previous two decades of gains in that group. This added weight to a 2005 study on a sample of 500,000 young Danish men, tested between 1959 and 2004, showing that performance peaked in the late 1990s but then declined to pre-1991 levels.

These findings conform with a pattern suggesting that the rise in IQ is slowing, and in some cases, going into reverse. In 2002, the psychologist Richard Flynn and political scientist Tatu Vanhanen co-authored IQ and the Wealth of Nations, with a follow-up in 2006, IQ and Global Inequality. The core message was that a nation's economic success was strongly correlated with its average IQ. Though controversial, IQ turns out to be a consistent indicator of success for individuals, nations and also companies - Microsoft co-founder Bill Gates once described his company as being in the IQ business.

According to the Flynn argument, a six-point decline in IQ would equate with a 0.3 per cent fall in GDP. This could also have social implications. A 2011 study by Intelligence linked average IQ to crime levels - though this correlation could be mediated by the possibility that falling IQ decreases prosperity, which causes an increase in crime.

The search for the Flynn effect's scientific basis has proved just as controversial as using IQ to measure intelligence. The Flynn effect has been strongest among people of average intelligence or below, and weakest among the very bright, suggesting a trickle-down effect, or redistribution of intelligence. Or perhaps government strategies to reduce the class divide by closing the IQ gap have been successful. In countries such as Britain where the Flynn effect appears to have been reversed, the turnaround came among higher-scoring people first.

No cause for this fall in IQs has been established: the internet, the dumbing down of education, and an obsession with exam results have been suggested. Flynn has argued that youth culture has made a contribution. Although it is not certain that the British and Danish results would be emulated elsewhere, falling IQ is a significant emerging trend. Others contend that the Flynn effect has reached its upper limits and that diet or education can only pull average IQ up so far. Scores cannot rise indefinitely - this would create whole nations of geniuses.

Tenuous evidence suggesting that youth culture or education plays a role emerged from a UCL study published last year. Contrary to earlier belief, it suggests that IQ among individuals can fluctuate during adolescence. This was a small study - 33 pupils aged 12 to 16, including high and low achievers. Their IQs were tested in 2004, and again three to four years later. Results found that while the average IQ of the sample remained the same, among individuals it fluctuated by up to 21 points.

Although taken from a small sample, the findings were reinforced by the most novel aspect of the study: brain scans were also taken using magnetic resonance imaging (MRI). This identified a correlation between the variations in IQ scores and changes in aspects of the brain including nerve density. It is not known how these changes relate to IQ, but the existence of an observable correlation between brain structure and intelligence could stimulate research into how cultural and educational factors influence neurological development.

Perhaps this should not be too surprising; it is known that exercise in childhood leads to permanent physiological changes and long-term benefits including increased lifespan. It is likely that, at a molecular level, similar mechanisms are involved, and this is the subject of the expanding field of epigenetics. This covers mechanisms that forge a biological link between nature and nurture, with cultural and educational factors modifying the activity of genes. Epigenetic mechanisms do not alter the underlying genes. This field does not, as is sometimes reported, challenge Darwinian evolutionary theory. Instead it adds a new layer of adaptation. Taking a piano as an analogy, if the keys are the genes, the pianist is the epigeneticist, who decides which ones to bring into play at various times, determining their expression. Occasionally some of the keys are silenced, so that they cannot be played, but in principle can be recovered for future use.

In some cases, neurological changes, caused by exposure to stimulation while young, are heritable. This aspect of the field - the inheritance of epigenetic characteristics - is poorly understood and not universally accepted, but there is research demonstrating its existence. Among the most convincing came in a study, published by Neuroscience in 2009, in which mice were genetically manipulated to impair their memory and then divided into two groups. One group was given stimulation to encourage motor skills. The other was not. Mice in the first group largely overcame their genetically programmed memory deficiency, and passed their improvements to offspring.

While research on human intelligence will always court controversy, there should be no moral or political objection to work examining how neurological development is directed by external factors and what affect these have on health and wealth.

Moreover, when it comes to intelligence, nature may create its own nurture. People inherit a desire to exercise their brains, and this stimulates neurological development. If love of thinking is the key to intelligence - at least of the kind measured by IQ tests - then perhaps there does need to be a change in education, especially in the early years, to develop intellectual curiosity. Then the necessary business of passing exams may take care of itself, and people will no longer leave school feeling, as Oscar Wilde did, that only then can their education begin.