A Studies from numerous countries suggest that IQ scores have been rising fast since at least the 1950s - enough to mean that someone with an IQ classed as average at that time could be labelled as having low intelligence today. However, psychologists disagree about what the upward trend in scores really means. Some researchers argue that people are not getting more intelligent overall, but just getting better at a particular sort of problem solving. Others say they have found echoes of rising IQ scores in 'real world' measures of intelligence. To confuse things still more, there are hints that the trend may be about to grind to a halt.

B James Flynn, a professor of political science, first showed that IQ scores were rising in the mid-1980s, and became famous after others dubbed the pattern he had observed the 'Flynn effect'. In a landmark paper he made detailed comparisons across fourteen countries of trends in the results of tests that measure reasoning power. Overall, these showed a real growth in scores of up to 25 points in a generation. To find out more, Flynn went on to look at scores for each type of reasoning that IQ tests set out to measure, such as verbal, numerical and visuo-spatial. This last test category usually includes 'Raven's matrices' - sequential patterns with one piece missing. The candidate has to choose, from a set of alternatives, which shape to put in the empty space. When Flynn took a closer look at the scores, the pattern was clear. The strongest gains in almost every case were on Raven's matrices and similar tests, while verbal and arithmetical scores showed more modest rises. Since presenting his original paper, Flynn has looked at data from further countries and found the same upward trend, with the same skills responsible for most of the increase.

C Flynn feels that his eponymous effect is meaningful, but does not interpret it as evidence that people are becoming cleverer. Our grandparents were not retarded and we are not geniuses, he says. More likely we have simply developed the skills and habits of mind that make us better at solving abstract problems, and - just as importantly - we take such problems seriously. Flynn believes that demands on visuo-spatial abilities have grown because of TV, computers and cars, and also that in societies where intelligence has become something people focus on excessively, abstract problem-solving skills develop faster than other skills.

D Others have different views about what might account for the increase in IQ scores. John Rust of the University of London suggests that it is not just an increase in visuo-spatial activities that is pushing up IQ scores. He believes that there are several factors, including the fact that, in more complex societies, people have to think harder to find solutions to problems they face. He also points out that if just one mind grasps a new concept, such as Einstein and the theory of relativity, it may then be easier for society in general to think in a more complex way. Others, such as experimental psychologist Robert Howard, argue that overall intelligence really has risen, helped by better nutrition, smaller families, wider access to education and other environmental changes.

E Howard's latest research has sent him on a different line of enquiry. Controversially, he now believes that the rise in IQ scores might have peaked in industrialised countries. To support this view, he cites data from Denmark. The Danish military still summons virtually all young men before a draft board to identify who is suitable to do military service. Since 1957, all 17-year-olds there have taken exactly the same qualifying IQ tests - including ones similar to Raven's matrices. Candidates must also be able to answer verbal questions such as 'Sun is to day as Moon is to ?', and number-series questions such as '2, 3, 5, 8, ?'. Results in these tests have been carefully analysed by Thomas Teasdale, a neuropsychologist. He found that during the 1960s and '70s, scores showed gains on a scale similar to those of other countries, but by the 1990s the rate of improvement had slowed right down to about one IQ point over the decade. Furthermore, the scores were improving only on the visuo-spatial questions; verbal and numerical performance was flattening out. Most intriguingly, however, scores from 1999 onwards show an overall downturn. One explanation might be that there is some absolute ceiling to people's abilities to perform these tasks, no matter how much the prevailing culture encourages them. However, as Teasdale points out, if such a ceiling had been reached, the test results should show scores pushing against it rather than actually falling.

F Flynn is keeping a close eye on the findings, but is not jumping to conclusions until data from other countries is available. However, he does wonder whether the conditions leading to dramatic improvements in mental ability disappear as societies become more prosperous. He points out that in human history, prosperity often led to decadence, and argues that the case of ancient Rome can be viewed as an illustration of this.

A The New Zealand species of lizard, the tuatara, is firmly embedded in the national psyche: an icon for today which dates from the age of dinosaurs; an ancient reptile commemorated on the back of the five cent coin. New Zealanders feel an affinity with the tuatara, and accept that active conservation management is required to ensure it will be among the legacies left to future generations.

B When European explorers reached New Zealand in 1769 they found two large islands, which together they called the 'mainland', and many tiny offshore islands around the coast. The naturalists who came with the explorers disregarded the tuatara, though it is improbable none were seen. Only several decades later did a tuatara specimen reach the British Museum, where it was eventually classified as just another type of lizard.

C One of the first scientists who realised that aspects of tuatara anatomy were odd – unchanged for tens of thousands of years – was Albert Gunther in 1876. Gunther believed the tuatara was one of the most valuable objects in zoological anatomical collections, and also noted, in passing, the reptile was likely to become extinct. From today's perspective, it is striking that Gunther expressed no concern about the probable demise of the tuatara. He and his contemporaries were products of their age, strongly influenced by Charles Darwin's theory, which had only recently been published. Their views were something like this: 'Extinction is a natural process. It is sad that species disappear, but that is part of nature.'

D There is a second important aspect of Gunther's work. He recorded, correctly, that some of the mammals introduced by Europeans were predators of the tuatara, particularly rats. But what he did not realise was that New Zealand has two species of rat, both introduced, both with an appetite for tuatara: the ship's rat came with European explorers and settlers; but the kiore rat had already been in the country for hundreds of years, brought by Polynesians from the Pacific Islands. Gunther failed to recognise the distinction, believing all rats to be a relatively recent introduction.

E Little further research was conducted until Ian Crook of the NZ Wildlife Service published his findings in 1973, which can be summarised as follows. Tuatara thrive on offshore islands with no rats. Tuatara never survived on islands with ship's rats. On a few islands, small and declining populations of tuatara occur with the kiore. This should not be seen, however, as evidence that tuatara and kiore can coexist. Rather, Crook proposed, kiore probably only arrived recently on such islands, and thus the small populations represent extinctions in progress.

F Throughout the 1990s, Richard Holdaway and his colleagues at Victoria University in Wellington documented the surprising discovery that kiore probably arrived about 1800 years ago, although the human population of New Zealand is thought to be no older than 800 years. How is this possible? Presumably, Holdaway argued, the kiore were brought by Polynesian explorers who visited the country but did not settle. Thereafter, the rats were agents of ecological warfare, exterminating perhaps 1000-3000 species. Thus, tuatara and many other species were already rare or extinct when permanent human inhabitants – the Maori – arrived around 1300. This hypothesis is still being debated, but the evidence continues to accumulate in its favour.

G Conservation practice has changed dramatically since Crook's findings were published in 1973. Eradication of rats from any given environment was believed to be virtually impossible until about 1980, but since then it has become routine. Enormous conservation benefits are accruing as newly rat-free offshore islands are providing sanctuaries for the country's rarest species. In 1995, for example, Nicola Nelson of the Department of Conservation established 68 tuatara on Titi Island. Since then, four more populations of tuatara have been established elsewhere under similar conditions. Today, numbers of tuatara are still a fraction of what they once were, but for the first time in 1800 years the decline has been reversed.

H While the recovery of rare species is itself a good thing, the truly significant outcome of this research is that it liberates the imagination. If we can remove predatory introduced mammals from islands, why not from the mainland too? Perhaps the questions we ask should demonstrate even more visionary ambition. Can non-mammalian pests also be removed from the mainland? Our rivers, for example, are full of surrogate rats, in the form of introduced species of fish called trout. Some day more people will understand that trout have replaced a whole native fauna in our waterways, just as rats replaced tuatara on the mainland. Will such knowledge lead to the creation of mainland 'aquatic islands' where we can once again establish those species of indigenous fish that used to live in our rivers? Similarly, can bellbirds and tuis replace birds like starlings and mynahs?

I The answers to such questions are uncertain, and opposing sides will doubtless be fiercely debated. But the role of scientific knowledge in illuminating the past will be crucial. Just as we now no longer tolerate extinction, in the future we may no longer accept a mainland devoid of the biological wonders of our past such as tuatara. Conservation is thus not primarily about the past but about imagining and then creating the future we wish for our children and ourselves. For 80 million years until humans arrived, tuatara occurred throughout New Zealand – might they do so again?