Psychiatrist Alfred Adler, a contemporary of Sigmund Freud, was convinced that our place among our siblings influences what he termed "style of life". Eldest children, he argued, are more likely to be neurotic and authoritarian as a result of younger siblings displacing them from their king-of-the-castle position and burdening them with extra responsibilities. Youngest children are spoiled and lack empathy; only middle children are even-tempered and successful, albeit more rebellious and independent, he asserted. Perhaps it was no coincidence that Adler himself was the second child of seven.
His thinking struck a chord. Interest in birth order and its possible consequences grew rapidly in the early 20th century, spawning a new field of research. In the 1980s, there was a backlash against the idea and most of the early studies have now been discredited. But in recent years, the pendulum has swung back, with compelling research revealing the importance of birth order in animals. Now there is mounting evidence that we, too, are influenced by our position in the family hierarchy. This appears to be linked with all manner of things, from body shape and intelligence to disease susceptibility and sexuality. The reason it has such far-reaching repercussions is also becoming clearer as we untangle the complex web of factors involved.
Historically, the inheritance of firstborns has often extended beyond their genes. Even before Adler came along, Charles Darwin's half-cousin, the eminent anthropologist, geographer and statistician Francis Galton, had claimed that it was exclusively firstborn males and only sons who went on to become renowned English scientists. It sounds like an outrageous generalisation but, in 1874, when his book English Men of Science: Their Nature and Nurture was published, there may well have been something to it. Back then, firstborn sons were often favoured when inheriting family wealth, giving them greater freedom to pursue the career or interests of their choosing.
In some families, even now, the firstborn may inherit the crown jewels or the family business - but, in general, cultural expectations based on family hierarchy are not as rigid as they once were. Nevertheless, Galton's work is highly relevant to modern researchers because of its emphasis on both "nature and nurture" - a phrase he coined. Distinguishing between biological and environmental factors is vital if we are to understand why someone's place among their siblings might affect his or her life chances. As a result, today's studies, unlike many in Adler's time, take account of factors such as socioeconomic status and family size. But what have they found?
One of the most striking discoveries to emerge from animal studies is that birth order can have life-or-death consequences. The same appears to be true in humans. A study of more than 600,000 people in Norway, by Hans Gravseth and colleagues at Oslo's National Institute of Occupational Health, found that the more older siblings someone had, the greater that individual's risk of committing suicide. The effect was more pronounced among women, although their suicide rate was one-quarter that of men.
"If you are a firstborn, the first few years you are alone, and have full attention from your parents. You may develop your personality in a more robust and stable direction, and develop resilience to stressful conditions later in life," says Gravseth. That reasoning is similar to Adler's, although he concedes that it isn't totally clear what the link between suicide rate and birth order might be.
Psychologist Catherine Salmon, at the University of Redlands, in the United States, has explored how birth order might affect family relationships. She finds that firstborns and lastborns tend to have the closest relationships with their parents, whereas middle children have stronger relationships outside the family. She puts this down to middle children tending to receive less parental attention, which, she says, helps hone their skills as "friendship specialists".
Many effects of an individual's place in the family are not a matter of life and death, but to do with behaviour. Mark Mainwaring and Ian Hartley, at Britain's Lancaster University, found that when zebra finch chicks become adults, the youngest birds from a clutch are more likely to be adventurous than their older siblings when exploring novel surroundings. In humans, too, there are hints that birth order is linked to risk-taking. Frank Sulloway and Richard Zweigenhaft, at the University of California, Berkeley, found that younger siblings were more likely to participate in dangerous sports. And among 408 brothers who played professional baseball, the younger brothers were 10 times as likely to attempt the high-risk ploy of stealing a base - and three times as successful when they did.
So firstborns may be more cautious, but they may be slightly more intelligent, too. An IQ study involving nearly 250,000 Norwegian male army conscripts found that eldest brothers had, on average, a 2.3-point advantage over second brothers - a trend that continued down the birth ranking. But when the researchers, led by Petter Kristensen, at the University of Oslo, looked at males whose elder brother had died - in effect moving the surviving brother up the ranking - they found that these men had a higher IQ than the average for their original slot in the hierarchy. Their results suggest that what matters is one's social position rather than actual order of birth. Although these differences in IQ scores are small, Kristensen suggests they could affect the chances of getting a university place.
Birth order may also influence leadership style and potential. When researchers at Leiden University, in the Netherlands, surveyed 1,200 Dutch men and women in public office, they found 36 per cent were firstborns and 19 per cent lastborns. The eldest and youngest children in a family each make up about a quarter of the Dutch population, so firstborns are over-represented and lastborns under-represented among the country's politicians. However, when it comes to leadership that challenges the status quo, Sulloway says, it is a different story. His research suggests that later-borns are more likely to embrace revolutionary ideas or movements. Darwin, he notes, was the fifth child in his family; firstborns, meanwhile, were vastly over-represented among opponents of his theory of evolution.
Marked physical differences between siblings are apparent in a variety of animals, and birth order also appears to have morphological repercussions in humans. For a start, firstborns tend to be taller: 2.5cm taller on average than their siblings, according to recent research by Wayne Cutfield and colleagues at the University of Auckland, New Zealand. Other research found that firstborn males also have a larger waist size as young adults. Epidemiologist Darren Dahly at the University of Leeds, Britain, who co-authored this study, suspects these differences arise before birth.
"The utero-placental vasculature may not be as well developed in the earlier pregnancies," he says. As a result, the first child is less well nourished in the womb than later ones and tends to be lighter at birth. Individuals whose metabolism develops in an environment of scarcity and who subsequently experience plenty are thought to be at higher risk of obesity in adult life - an idea called the mismatch hypothesis, in reference to the mismatch between pre- and postnatal environments. Dahly believes this may help explain his findings.
This could perhaps be linked to another discovery by Cutfield and colleagues. Studying prepubescent children, they found that the ability of the body to respond to insulin was 21 per cent lower in firstborns than in later-borns, and that their blood pressure was significantly higher. They suggest that this may make firstborns more prone to adult diseases including type 2 diabetes, heart disease, stroke and hypertension. Birth order may influence our susceptibility to other diseases, too, including the types of cancer we are prone to - although the picture is still unclear.
A link with allergies is well documented, however. Matthew Perzanowski, of Columbia University, in New York, studied low-income children in the US government-funded Head Start programme. He found that four- to five-year-olds with older siblings were almost three times less likely to go to the emergency room with allergic asthma than children without older siblings. According to what is called the hygiene hypothesis, being exposed to more viruses, fungi and bacteria early in childhood somehow tempers the immune system, making later-borns, whose older siblings might bring pathogens into the home, less prone to allergies than firstborns.
An alternative explanation links allergies and asthma to the fetal environment. The idea is that the mother's body must downgrade its immune response to avoid rejecting the developing child. Perzanowski suggests that if this becomes more effective with each successive pregnancy, then later-borns will develop fewer antibodies against their mother in the womb, making them less likely to overreact to innocuous substances they encounter in the outside world.
But this idea runs counter to another fascinating insight. In 1992, Ray Blanchard, at the University of Toronto, Canada, showed that the more older brothers a man has, the more likely he is to be gay. He suggested that male fetuses trigger an immune response in their mother, one that becomes stronger with each pregnancy. A link between homosexuality and being a younger brother has since been found in many populations.
As for the underlying mechanism, Tony Bogaert at Brock University, in Hamilton, Canada, suspects that the target of the mother's immune response may be proteins on the surface of male fetal brain cells in the anterior hypothalamus, a brain area linked to sexual orientation. If her antibodies bind to these molecules and alter their role in typical sexual differentiation, that might lead some later-born males to be attracted to men, he suggests. Since blood retains an immunological "memory" of past immune responses, even after many years, Bogaert is now analysing blood samples from mothers of gay and straight sons to test this idea.
Birth order is a flip of the evolutionary coin, and just one component of the multiplicity of factors that make us who we are as individuals. But a decline in fertility rates worldwide means that the proportion of people who are firstborn children is rising, making it more important to understand what impact birth order may have. It may subtly influence our physical and mental health, our opportunities for education and our careers. Not all younger siblings will be spoiled and allergy-free, not all middleborns will be social butterflies, and not all older siblings will be tall, intelligent, responsible leaders, but our place in parity provides fascinating insights into the complexity that makes each of us unique.
In some birds, birth order is a matter of life or death. Nazca boobies, seabirds that breed on the Galapagos Islands, lay two eggs. The first-hatched nestling always attacks its younger, smaller sibling, finally ejecting it from the pebble-strewn nest. Mother boobies do not intervene, leaving the expelled chick to face certain death from dehydration or predators. In the related blue-footed booby, both chicks may survive when food is plentiful but, if it is scarce, the older chick shows no mercy, attacking and killing its sibling.
Scott Forbes recalls being shocked when he first observed baby ospreys "beating the hell out of each other" during his doctoral research. Now at the University of Winnipeg, Canada, he studies sibling rivalry in red-winged blackbirds. Although not quite as dramatic as in ospreys, it is lethal all the same. In a typical brood of five eggs, he says, "the last-hatched chick has roughly a 10 per cent chance of surviving, while the first-hatched chick has a better than 80 per cent chance".
Clutches of blackbird's eggs, like those of many birds, do not hatch simultaneously, and this appears to provide a safety valve. When times are good and food is plentiful, even the runts may survive. When times are tough, it is survival of the fittest, and older chicks almost always win.
In one experiment, Forbes manipulated the eggs of yellow-headed blackbirds so that they all hatched simultaneously. It was disastrous for the offspring: in lean years, all the chicks died. He likens the phenomenon to investing money, with blackbird parents dividing their offspring into core and marginal groups. The older siblings are the "blue chips", the younger ones their more risky holdings that only sometimes pay Darwinian dividends.
It is not just birds that play birth-order politics. The eldest piglets in a litter use their temporary teeth to fight their younger siblings for access to their mother's frontmost teats, which provide the most milk. By achieving this position, they are more than twice as likely to survive than siblings further down the milk bar.
In sand tiger sharks, siblicide occurs before birth, with the largest embryo in each of the mother's uteri breaking out of their egg capsules and engaging in cannibalism of smaller siblings.
"Birth order" even matters in some plants. Indian black plums produce seeds with up to 30 ovules, but the first to be fertilised secretes a toxin that kills off the rest. New Scientist