经济学人64:双胞胎母亲更幸福

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　　拥有双胞胎的母亲不仅能得到双份的快乐，而且她们会比其她母亲更健康。

　　THROUGHOUT history, twins have provoked mixed feelings. Sometimes they were seen as a curse—an unwanted burden on a family’s resources. Sometimes they were viewed as a blessing¹, or even as a sign of their father’s superior virility². But if Shannen Robson and Ken³ Smith, of the University of Utah, are right, twins have more to do with their mother’s sturdy constitution than their father’s sexual power.

　　在历史上，对待双胞胎有着复杂的感情。有时，他们被视为祸根--家庭中不受欢迎的负担。有时，他们被视为祝福，或作为他们父亲拥有更好的性能力的一个标志。但如果犹他大学的夏侬.罗布森和肯.史密斯是正确的，那么双胞胎为他们的母亲那健壮的体格所做的比为他们的父亲的性能力方面所做的更多。

　　At first blush, this sounds an odd idea. After all, bearing and raising twins is taxing, both for the mother and for the children. Any gains from having more than one offspring at a time might be expected to be like higher infant and maternal⁴ mortality rates. On this view, twins are probably an accidental by-product⁵ of a natural insurance policy against the risk of losing an embryo⁶ early in gestation⁷. That would explain why many more twins are conceived than born, and why those born are so rare (though more common these days, with the rise of IVF). They account for between six and 40 live births per 1,000, depending on where the mother lives.

　　乍看之下，这好像听起来好像是一个奇怪的想法，毕竟对于母亲和孩子，生产和抚养双胞胎是很费力的。每次从多于一个孩子那里获得的比期望的更多，通常会伴随着较高的婴儿死亡率和母亲死亡率。根据这种观点，双胞胎很可能是一次在怀孕期间用自然保障方式防止流产的意外的副产品。这样就能解释为何双胞胎怀的比生的多，而且为什么生的会这么少（虽然现在更多的运用试管受精已经很平常）。他们解释说每1000个中成活率在6到40之间，这要根据母亲生活的地方。

　　Dr Robson and Dr Smith, however, think that this account has got things the wrong way round. Although all women face a trade-off between the resources their bodies allocate⁸ to reproduction and those reserved for the maintenance of health, robust⁹ women can afford more of both than frail¹⁰ ones. And what surer way to signal robustness¹¹ than by bearing more than one child at a time? In other words, the two researchers conjectured¹², the mothers of twins will not only display greater overall reproductive success, they will also be healthier than those who give birth only to singletons.

　　然而，罗布森博士和史密斯博士认为这个解释是事情处于错误中。尽管许多女人面临着一个权衡，那就是在身体分配给生育所需的资源和保留给保持身体健康所需营养之间权衡。强健的母亲可能比瘦弱的母亲提供的更多。而且能有什么比健壮的母亲生育多于一个孩子更有说服力呢？换言之，这两位调查者推测，双胞胎的母亲不仅更好的展示了整体的生育成功，也会比只生一个孩子的母亲更健康。

　　Alas¹³, pinning down evolved relationships between fertility and health is tricky¹⁴. Modern medicine and the pampering¹⁵ effects of economic growth mean that, these days, women everywhere give birth to fewer children than they did in the distant evolutionary¹⁶ past, when human bodies and physiology¹⁷ were forged—even as more of the offspring they do bear survive into adulthood¹⁸. In Europe and North America this demographic transition began in earnest around 1870.

　　唉，控制生育和保持健康之间的关系是很难的。现代医学和经济的发展精心呵护的结果是，当人体和生理机能被伪造，这个时候的女性无论在哪里都会比处在以前缓慢发展时期的女性生的孩子要少，因为她们生的孩子大部分能活到成年。在欧洲和北美这个人口过渡开始于1870年左右。

　　One way round this is to look at records gathered before the demographic transition began. The problem is that few authorities then kept tabs diligently¹⁹ enough to provide adequate fodder²⁰ for statistical²¹ analysis. One exception was Utah, a state largely settled in its early days by Mormons, who have always taken such matters seriously. As a result, Utah’s Pedigree and Population Resource, a database of which Dr Smith is the director, contains 1.6m genealogical records for people in the state from the early 1800s to the 1970s.

　　围绕着寻找记录及合并的方法要在人口过渡开始前完成。这个问题是只有很少的当局会勤奋的为统计分析提供足够的材料保持制表符。一个例外是犹他州，在早期有摩门教徒定居在此，他们总是严肃的对待事情。结果，在史密斯博士作为主管的一个数据库中，犹他州的血统和人口资源包括160万份在早期的1800世纪到1970世纪中居住于此的居民的家谱记录。

　　To test their theory that the bearers of twins are supermums, Dr Robson and Dr Smith scoured²² the records for women born between 1807 and 1899 who survived past the age of 50, when the menopause typically strikes. They excluded those married more than once, those widowed before they were 50 and also (this being Utah) those wives who were part of one of the polygamous marriages once legal among the state’s Mormon settlers. (In a study published earlier this year, Dr Smith and his colleagues found that the more wives a Mormon woman had to compete with for her husband’s attentions, the fewer children she was likely to produce.) This left them with some 59,000 women, around 4,600 of whom had given birth to twins at least once.

　　为了检验他们关于双胞胎母亲是超级妈妈的说法罗伯森博士和史密斯博士搜集了关于超过50岁的更年期正常来临并在1807年到1899年进行生产的妇女的资料。她们只结过一次婚，在50岁以前就守寡并且这些妻子是一夫多妻制婚姻的一部分，而这在这一地区的摩门教徒中是合法的。（今年早期发布的一项研究表明，史密斯博士和他的同事发现在众多妻子中，一个摩门妇女不得不为了吸引丈夫的关注而竞争，这样她很可能会生更少的孩子。）这样的妇女大约有59000名，其中4600名至少一次的生产双胞胎。

　　To assess the effects of the demographic transition, the two researchers split the sample into women born before 1870 and those born in or after it. The results, just published in the Proceedings²³ of the Royal Society, confirm their hypothesis. Mothers of twins in the pre-1870 batch²⁴ gave birth, on average, to 1.9 more children than the mothers of singletons. Those in the post-1870 batch had 2.3 more. More importantly, from an evolutionary point of view, more of their children made it to maturity²⁵. The mothers of twins in the pre-1870 batch had 1.24 more children who survived past the age of 18 than did other mothers. After that date the figure was 1.56.

　　为了评估人口过渡的影响，这两名研究员分头对在1870年前和在这时及之后进行生产的妇女进行取样。结果公布在英国皇家学会学报上以证明他们的假说。在1870年前生产双胞胎的母亲平均比生一个的多生1.9个孩子。这在1870年后就变为多于2.3个了。更重要的是，从一个逐渐发展的角度来看，她们的孩子会更成熟。比起其她母亲来说，在1870年以前生育双胞胎的母亲会有多于1.24个孩子，这些孩子更能活到18岁。之后这个数据变成了1.56。

　　Of course, having twins automatically increases a mother’s lifetime fertility by one, relative to her number of pregnancies²⁶. But, since few women have twins more than once, the data imply that mothers of twins are more productive, even ignoring that effect. Moreover, they also outperformed the mothers of singletons on a range of other reproductive measures, including intervals²⁷ between births (shorter) and the age at which they gave birth to their last child (older).

　　当然，拥有双胞胎无意识的增强了母亲一生的生育力，这也和母亲的怀孕时间有关。但是如果很少妇女能多于一次的生产双胞胎，那么数据显示双胞胎的母亲拥有更好的生育力，甚至可忽视其影响。此外，她们也超过一个母亲的其她生育价值，包括生育之间的间歇和生育最后一个孩子的年龄。

　　Most strikingly, the pre-1870 group of mothers of twins actually lived longer than those who were not thus blessed. They were, in other words, simply healthier. This suggests that, in evolutionary terms at least, twins are far from a curse. On the contrary, they are indeed the mark of a supermum.

　　更明显的，在1870年前生产双胞胎的母亲实际上比没有这份祝福的要活得更长。换言之，她们是更健康的。这个建议说明，至少在进化过程中，双胞胎不是祸根，正好相反，他们却是超级母亲的标志。

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