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Does your older brother think he’s cleverer than you? Well, he’s probably right. According to new research due to be published this week in the journal Intelligence, the oldest children in families are likely to have the highest IQs, and the youngest the lowest.

The research is based on more than 1,000 children whose IQ was tested through childhood and adolescence up to the age of 18. The Dutch study shows a birth-order effect on intelligence in each of the tests. Overall, the IQ of the first-born child was higher than the second-born, which, in turn, was greater than that of children who had two or more older siblings.

This is only the latest research to suggest that the order of birth can have a fundamental effect on diverse factors, ranging from the risk of cancer, asthma and eczema, to weight and even premature death.

It can also affect personality, achievement, and career, with first-borns being more academically successful and more likely to win Nobel prizes. However, eldest children are less likely to be radical and pioneering. Charles Darwin, for example, was the fifth child of six.

It has even been suggested that birth order can influence sexual orientation, left or right handedness, and the number of sexual partners someone has in a lifetime.

Exactly why there should be such differences is not clear, and there are a number of theories, with many homing in on environmental influences on the child.

The so-called dilution hypothesis suggests that as family resources, both emotional and physical, as well as economic, are finite, it follows that, as a result, as more children come along, the levels of parental attention and stimulation will drop. Another theory is that the intellectual environment in the family favours the first-born who has, at least for some time, the benefit of individual mentoring

Here are some of the factors that scientists believe may vary with birth order, and why.

INTELLIGENCE

A number of studies have suggested that IQ scores decline with birth order. In the most recent study, at Vrije Universiteit, Amsterdam, researchers looked at men and women whose IQ had been tested at the ages of 5, 12, and 18.

The results, which show a trend for the oldest to score better than the youngest in each test, confirm the findings of a study at the University of Oslo, involving about 200,000 people. That showed that first-borns had a three-point IQ advantage over the second-born, who was a point ahead of the next in line.

The theory which enjoys the most support is that the extra time and patience that the earlier borns get from their parents, compared with those arriving later, gives them an advantage.

PERSONALITY

This is one of the most researched areas. A study at the University of California (and several other institutions) based on more than 2,000 families from six countries, suggests that the parents’ most favoured child tends to be the last-born. The rebel of the family also tends to be born later that his siblings, but he will not necessarily be the last-born, and rebels tended to feel less close to their parents

First-borns are . . .achievers, who are dominant, religious, conscientious and neurotic. They earn more, are more responsible, anxious and organised, and they stick to the rules.

Middle-borns are . . .rebellious, less religious, impulsive and open to new experiences. They perform worse at school and often procrastinate but act as peacemakers.

Last-borns are . . .agreeable, warm, sociable, extrovert and creative. They are the most favoured child, often a joker and questioning of authority.

BROTHERLY LOVE

One theory of sibling relationships suggests that older siblings invest more time and effort in younger ones than vice versa. To test the theory, researchers at Newcastle University looked at whether first-borns were more likely to keep in touch with their siblings than middle-borns or later-borns, based on a sample of 1,558 people.

First-borns were found to have significantly more frequent face-to-face contact, every week with their siblings than middle-borns or last-borns, even after taking into account geographic distances. Middle-borns and later-borns were less likely to have frequent contact with each other.

MORTALITY

Later-borns are more likely to die prematurely. A study that followed 14,000 boys and girls born between 1915 and 1929 until they died shows that even when birth weight, gestational age, diseases, social class and other factors are taken into account, the youngest born have a higher risk of mortality. “The general tendency was for later-born siblings, particularly girls and women, to demonstrate a higher mortality risk than first-borns,” say the researchers from Stockholm University.

One possible explanation is that later-borns are also associated with greater risk-taking.

ECZEMA AND ASTHMA

Eczema and asthma are some of the most common chronic childhood diseases and research has suggested that later-borns have a reduced risk. A study last year by Dr Paolo Matricardi and colleagues in Rome and Napoli, and based on 11,371 young men, showed that the prevalence of eczema and asthma was related to the total number of siblings. The fewer siblings you have, the greater your chance of having asthma or eczema. One theory is that younger children are exposed to a wider range of infections by their older siblings and this helps to educate their immune systems and protect them.

SEXUAL PARTNERS

Later-borns want to enjoy more sexual partners than first-borns. When researchers from Florida Atlantic University questioned fellow college students, they found that those who were the youngest in the family desired more sexual partners, but that the first-borns wanted to have children at an earlier stage than later-born siblings.

That, they claim, suggests that a greater pursuit of a long-term mating strategy by first-borns. The theory is that first-borns uphold the values of their parents, which are more likely to result in having children early, pursing fewer sexual partners and wanting a long-term relationship. Later-borns often play a role of rebels, who pursue different paths.

WEIGHT

Later-borns are less likely to be overweight, according to a study based on 8,000 school children. Researchers at the University of Toyama in Japan found that the risk of being overweight in boys in particular was significantly lower with increasing numbers of elder siblings or a sister. They also found that boys from three-child families had a significantly lower risk of being overweight than only children. Just why is not clear, but one theory discussed by the researchers is that over-protection and overfeeding are probable mechanisms leading to obesity because, they say, mothers are more concerned with persuading children to eat in small families. Another possible mechanism is that there is less food for each child in large families