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2016年04月27日11:19 来源:小站整理
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Mothers of twins do not just get twice the bundle of joy—they are also healthier than other mothers


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 blessing, or even as a sign of their father’s superior virility. But if Shannen Robson and Ken 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.



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 maternal mortality rates. On this view, twins are probably an accidental by-product of a natural insurance policy against the risk of losing an embryo early in gestation. 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.


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 allocate to reproduction and those reserved for the maintenance of health, robust women can afford more of both than frail ones. And what surer way to signal robustness than by bearing more than one child at a time? In other words, the two researchers conjectured, 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.



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


One way round this is to look at records gathered before the demographic transition began. The problem is that few authorities then kept tabs diligently enough to provide adequate fodder for statistical 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 scoured 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.



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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 Proceedings of the Royal Society, confirm their hypothesis. Mothers of twins in the pre-1870 batch 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 maturity. 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 pregnancies. 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 intervals between births (shorter) and the age at which they gave birth to their last child (older).


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.




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