你哥哥认为他比你聪明吗？哼，他可能是对的。根据一项发表在《智力》杂志上新研究，家庭中最大的孩子智商可能最高，最小的智商最低。

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 adolescence¹ 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 siblings².

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, asthma⁴（哮喘） and eczema（湿疹）, to weight and even premature⁵ death.

It can also affect personality, achievement, and career, with first-borns being more academically successful and more likely to win Nobel prizes. However, eldest⁶ children are less likely to be radical⁷ and pioneering. Charles Darwin, for example, was the fifth child of six.

It has even been suggested that birth order can influence sexual orientation⁸, 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 dilution⁹ 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 parental¹⁰ attention and stimulation¹¹ will drop. Another theory is that the intellectual environment in the family favors the first-born who has, at least for some time, the benefit of individual mentoring¹²

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 favored 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 dominant¹³, religious, conscientious¹⁴ and neurotic¹⁵. They earn more, are more responsible, anxious and organized, and they stick to the rules.

Middle-borns are . . . rebellious¹⁶, less religious, impulsive¹⁷ and open to new experiences. They perform worse at school and often procrastinate¹⁸ but act as peacemakers.

Last-borns are . . . agreeable, warm, sociable¹⁹, extrovert²⁰ and creative. They are the most favored child, often a joker and questioning of authority.

BROTHERLY LOVE

One theory of sibling³ relationships suggests that older siblings invest more time and effort in younger ones than vice²¹ 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 geographic²² distances. Middle-borns and later-borns were less likely to have frequent contact with each other.

MORTALITY

Later-borns are more likely to die prematurely²³. A study that followed 14,000 boys and girls born between 1915 and 1929 until they died shows that even when birth weight, gestational（妊娠期的）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.

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 mechanisms²⁴ leading to obesity²⁶ because, they say, mothers are more concerned with persuading children to eat in small families. Another possible mechanism²⁵ is that there is less food for each child in large families.