生物谷报道：对挪威青少年进行的一项最新研究显示，兄弟姐妹之中老大的智商最高，他们的IQ测试得分往往比弟弟妹妹们高出2.3分。科学家认为，这一现象并非源于生理因素，而是所谓的“家庭动力学”的结果。相关论文发表在6月22日的《科学》杂志上。
长期以来人们就认为如果从足够大的人口样本取样，长子或长女比弟弟妹妹们更聪明。在现实生活中，很多“老大”在社会上地位显赫。一些研究甚至表明，出生次序与智商之间存在着某种关系：越晚出生，智商越低。
不过，许多人对这一关系的可信度表示怀疑，至于其背后的深层原因更是众说纷纭。有些批评家认为，智商相对较低的家庭往往比较庞大，兄弟姊妹众多使得每个人获得功课辅导的时间较少，因此，越晚出生的孩子往往越不聪明。
在最新的研究中，挪威奥斯陆大学（University of Oslo）的Petter Kristensen和挪威武装军队医学服务机构（Norwegian Armed Forces Medical Services）的Tor Bjerkedal对241310名挪威青少年进行了研究，他们的年龄都是18或19岁，很适合进行智商检测。
研究人员发现，头胎孩子的平均IQ得分超过了103，而第二胎孩子仅超过100，第三胎约为99。然而，由于老大夭折而“晋升”成为老大的孩子，他们的平均IQ得分也能达到103。
研究人员在另一项单独分析研究中发现，这一总体趋势也适用于单个家庭，也就是说先出生的孩子智商更高。这项单独研究将发表在近期的《智力》（Intelligence）杂志上。
究竟是什么造成了同胞间的智商差异呢？Kristensen认为，这是家庭和社会因素造成的，比如父母对长子长女的关注较多，而且年长的孩子会花时间教育帮助弟弟妹妹等等。
Kristensen自身是第二个出生的孩子，他表示，一开始自己凭直觉的判断，不相信越早出生的孩子越聪明，尤其是还有研究表明，第二胎和第三胎孩子的健康状况往往比老大更好。
美国加州大学伯克利分校的Frank Sulloway表示，该研究结果并不能说明家庭中较小的孩子一定会受到智力较低的不良影响。他说，“许多证据已经表明，第一个出生和后来出生的孩子所擅长的是不同的事情。”比如达尔文，虽然他是六个孩子中第五个出生的，一些功课也不是很好，但他是一个有巨大好奇心的人。Sulloway说，“如果让我在IQ上多2.3分和拥有达尔文的巨大好奇心之间做出选择，我会毫不犹豫地选择后者。”（引自科学网 任霄鹏/编译）
原始出处：
Science， 22 June 2007:
Vol. 316. no. 5832, p. 1717
DOI: 10.1126/science.1141493
Explaining the Relation Between Birth Order and Intelligence
Petter Kristensen1,2* and Tor Bjerkedal3
Negative associations between birth order and intelligence level have been found in numerous studies. The explanation for this relation is not clear, and several hypotheses have been suggested. One family of hypotheses suggests that the relation is due to more-favorable family interaction and stimulation of low-birth-order children, whereas others claim that the effect is caused by prenatal gestational factors. We show that intelligence quotient (IQ) score levels among nearly 250,000 military conscripts were dependent on social rank in the family and not on birth order as such, providing support for a family interaction explanation.
1 National Institute of Occupational Health, N-0033 Oslo, Norway.
2 Section for Preventive Medicine and Epidemiology, University of Oslo, N-0318 Oslo, Norway.
3 Institute of Epidemiology, Norwegian Armed Forces Medical Services, N-0015 Oslo, Norway.
* To whom correspondence should be addressed. E-mail: petter.kristensen@stami.no
The interest in the relation between birth order and intelligence dates back to Sir Francis Galton's English Men of Science (1). Galton found more firstborn sons in prominent positions than what he attributed to chance. This was the start of numerous studies; one of the most influential was a Science publication in 1973 showing a negative association between birth order and intelligence in young Dutch men (2). Since then, sociologists, psychologists, and demographers have proposed several explanatory models (3). The most influential models have emphasized explanations relating to interactions within the family and favorable conditions for intellectual stimulation for low-birth-order children.
Several researchers have claimed that the relation between birth order and intelligence is false, confounded by factors relating to family size: Families with low-intelligence children tend to be large, and the relation with birth order is an artifact when comparisons between families are made (3). This explanation would not produce birth order effects between siblings. Thus, the demonstration of small but notable birth order effects on intelligence quotient (IQ) in large studies examining relations within families (4, 5) contradicts the idea that artifact is the full explanation.
A third model claims that the relation is explained by prenatal or gestational factors. One hypothesis suggests an effect of maternal antibody attack on the fetal brain: Maternal antibody levels tend to increase by higher birth orders in a suggested mechanism parallel to rhesus incompatibility and erythroblastosis (6). It has been shown that children of mothers with autoimmune disease have an increased risk of learning disabilities [for example, (7)], but there are no empirical data to support immunoreactivity in explaining the birth order effect.
