Amberley: IELTS Academic Reading practice test

Amber is fossilised tree resin, and the distinction between resin and sap matters more than it sounds. Sap is the fluid that carries water and sugars around a living tree, and it does not fossilise. Resin is something else entirely: a thick, sticky secretion produced in response to damage, loaded with compounds that kill bacteria and fungi and seal a wound. Over millions of years the volatile components of a lump of resin evaporate away and what remains polymerises into a hard, stable solid. The intermediate stage, in which the material has hardened but the process is incomplete, is called copal, and a great many pieces sold as amber are in fact copal that is a few thousand years old rather than a few million.

The reason amber preserves so extraordinarily well follows from what resin is for. A small animal caught in a flow is immobilised at once, and the resin then draws water out of its tissues while excluding oxygen and the microbes that would otherwise consume it. What remains after a hundred million years is a three-dimensional record of the animal's surface, accurate to a fraction of a millimetre: individual hairs, the scales on a moth's wing, the facets of an eye. Rock fossils give a flattened outline. Amber gives something closer to a photograph. It also catches behaviour, which almost nothing else in the fossil record does: specimens exist of insects mating, of a spider approaching a trapped wasp, of a parasite in the act of feeding, and in one celebrated case of a dinosaur-era bird's feathered tail.

It is necessary to be blunt about what amber does not preserve, because the popular understanding of it rests on a claim that turned out to be wrong. Reports published in the early 1990s described the recovery of genetic material from insects tens of millions of years old, and they were never successfully repeated. Careful subsequent attempts, using the contamination controls that the original work lacked, have recovered nothing at all. The reason is chemical rather than technical: even under ideal conditions the bonds in a strand of DNA break at a rate that leaves nothing legible after something in the order of a million years, and amber, contrary to what one might expect, does not slow this down.

Three deposits supply most of the material that scientists work with. Baltic amber, from around forty million years ago, is the most abundant and the source of the majority of specimens in European collections; the tree that produced it has never been identified with confidence. Dominican amber is younger, at fifteen to twenty million years, and came from a species of tropical legume that still exists. The most scientifically valuable is Burmese amber, which at about ninety-nine million years belongs to the Cretaceous, and is therefore old enough to record the world in which flowering plants were spreading and the modern insect orders were taking shape.

That third deposit has become an ethical problem of the first order. The mines lie in a region that has been the site of an armed conflict, the material is exported through channels that are difficult to trace, and the revenue does not obviously reach the people digging it. Several scientific journals and at least one major learned society now decline to publish descriptions of specimens acquired after a stated date, and others have imposed conditions on provenance rather than a ban. The argument within the field is genuinely difficult. A specimen that cannot be published is, for practical purposes, lost to knowledge; and a specimen that is published acquires a value that encourages the trade which produced it.

What amber has actually taught is considerable and comes with a caution attached. It has dated the origins of several insect orders far earlier than the rock record suggested, documented the relationship between flowers and their pollinators almost from the beginning, and preserved whole categories of animal that rocks lose entirely: mites, springtails, tiny parasitic wasps and other small soft-bodied creatures that leave no impression in sediment. The caution is that a resin flow samples a particular place. It catches what walks on bark and what falls from the canopy onto the forest floor, in forests of the kind that produce resin, and it tells you very little about anything that lived in open ground or in water.

Because the material is valuable, it is also faked, and the tests used are mostly simple. Genuine amber floats in a strong salt solution while most plastics sink. It fluoresces under ultraviolet light in a way that an imitation generally does not. A heated needle touched to a hidden surface produces a smell of pine from amber and of burning plastic from a substitute. The most troublesome forgeries are not plastic at all but young copal, or fresh resin with a modern insect deliberately embedded, and for these the only reliable test is an examination of the inclusion itself: an insect that belongs to a species alive today did not die a hundred million years ago.

It is worth pausing on what makes these objects unusual. A fossil is normally a mineral replica, a shape in stone that has to be interpreted. An inclusion in amber is the animal itself, or what is left of it, held in the position it was in on the afternoon it died, with the resin still preserving the direction it was trying to run. Very little else in the study of the past offers that, and it is not surprising that the material has attracted more speculation, more forgery and more bad science than any other kind of fossil.

Almost everybody has a view about birth order. The eldest is conscientious and anxious to please, the youngest is charming and takes risks, the middle child is overlooked and becomes a negotiator. The belief is held with remarkable confidence by people who would not claim to know anything else about personality, and it has a respectable scientific version. On that account, siblings compete for the attention of the same parents, and since the eldest occupies the obvious position of the responsible high achiever, later children must differentiate themselves by finding an unoccupied role, which tends towards the unconventional. It is an elegant theory and it fits everybody's family.

It also drew support from a substantial research literature, and the way that literature went wrong is worth understanding, because the error is subtle and was invisible for decades. Most of the early studies compared people from different families: a sample of adults was assembled, each was asked about their position among their siblings, and the groups were compared. The difficulty is that birth order and family size are not independent. If you select people at random, a first-born is more likely to come from a small family, because every family has exactly one, while fourth-borns can only come from families of four or more. Family size in turn correlates with income, parental education and a great deal else. Any comparison across families is therefore comparing social background as much as sibling position.

The correction, when it came, required data nobody had previously assembled: very large panel surveys containing several siblings from the same household, so that brothers and sisters could be compared with one another rather than with strangers. Within a family, the confound disappears. Studies of this kind, using tens of thousands of individuals across several countries, agree closely with one another and produce two findings. On measured intelligence there is a small but consistent advantage to being born earlier, on the order of one and a half points between a first and a second child. On personality, once the appropriate comparison is made, the effects are indistinguishable from zero. Not small. Zero.

The intelligence finding is real and requires an explanation, and there are three candidates. One is dilution: parental time and money divided among more children leave less for each, and the eldest has a period with none of that division. A second is tutoring: an older child who explains something to a younger one understands it better afterwards, and only the eldest has a full complement of younger siblings to teach. A third is biological, having to do with the changing uterine environment across pregnancies. The third can be tested against the other two, and the test is unusually clean. In a very large study of military conscripts, men who were biologically second-born but had been raised as the eldest because an older sibling died in infancy scored like first-borns rather than like second-borns. Whatever is producing the effect follows social position in the family, not the order of births.

Why, then, does everybody believe the personality version? Partly because families supply endless confirming instances and no disconfirming ones, since a family with an unconventional eldest is filed under exceptions. Partly because the theory explains things one already knows about one's own relatives, and an explanation that arrives after the fact is very hard to reject. And partly because the effects that do exist are far too small to be visible in a single case: a difference of one and a half points on a test with a standard deviation of fifteen is real across a hundred thousand people and completely invisible across a dinner table.

Not everybody accepts that the matter is settled. Tomas Wreford, who works on family dynamics, makes two objections. The first is statistical: a within-family design removes everything that families differ by, and some of what it removes may be exactly what birth order operates through, since a family's whole way of proceeding may be shaped by how many children arrived and in what order. The second is about measurement. The questionnaires used to assess personality in these studies produce five broad scores, and the folk theory is not really about broad traits at all; it is about roles, about who is expected to organise the holiday and who is forgiven for not turning up, and an instrument that measures conscientiousness on a nine-point scale will not detect that.

The second objection has some force and the first does not survive inspection. A within-family design does discard between-family variation, which is precisely what it is for, since that variation was the source of the original error; the proposal to put it back is a proposal to reinstate the confound. On measurement, Wreford is right that the instruments are coarse, and it is entirely possible that something real is going on which five broad scores cannot capture. But the burden sits with the claim. A theory that predicts effects too subtle for any available instrument to register has stopped predicting anything, and the correct response is to build the instrument rather than to treat the absence of evidence as a technicality.

What makes the episode worth studying is not the conclusion but the route to it. For half a century a large, active and perfectly competent research field produced consistent support for a hypothesis, and the support was an artefact of a design choice that everybody had inherited and nobody had examined. The studies were not fraudulent and the researchers were not careless; they were all making the same comparison, and the comparison was wrong. That is a more unsettling story than any individual failure, and it is the reason methodological arguments, which are tedious, deserve more attention than they get.