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  3. All over the world, more boys are born than girls. Evolutionary biologists believe that this is because boys are more likely to

All over the world, more boys are born than girls. Evolutionary biologists believe that this is because boys are more likely to

admin2012-03-23  67
问题     All over the world, more boys are born than girls. Evolutionary biologists believe that this is because boys are more likely to die at a given age than are their female contemporaries. The imbalance at birth thus means that the sex ratio balances at the age when people are reproducing. But for decades there has been a puzzling trend in the boy. girl ratio. In Britain, as well as in the United States and Canada, the proportion of boys being born is dropping. No one knows why, although it has been suggested, somewhat controversially, that the trend is due to chemical pollutants that are mimicking the effects of sex hormones.
    And yet there is another recent trend that may have something to do with it. During the same period, the proportion of single mothers has been increasing. The reasons {or this are less puzzling, but as the Italian nominee to the European commission, Rocco Battalions, found out when he apparently suggested that single mothers were not very good as parents, it is no less controversial. The question is, could the two trends be linked? Can household arrangements affect the human sex ratio?
    According to Karen Norberg, of the National Bureau of Economic Research in Cambridge, Massachusetts, they can Dr. Norberg found that the chance a woman giving birth to a boy rather than a girl is higher if she has been living with a man before the child was conceived. To be specific, for parents who were living together, boys were born 51. 50% of the time, while when the parents were not cohabiting only 49. 90% of births were male. This difference may seem small, but statistically it is highly significant, which suggests it is the result of evolution.
    Actually, as with so much else in biology, Charles Darwin got there first. In "The Descent of Man", he referred to studies showing that among children born out of wedlock there were more girls than boys. Dr. Nordberg’s work reinforces the point, and also shows that it is not formal marriage, but actual cohabitation, that is the decisive factor. What neither Darwin’s nor Dr. Nordberg’s work shows, though, is why.
    There are some clues. In work on other mammals, researchers have found an association between hormones, the frequency of copulation, and the sex of the of fspring. In other words, there is a way the body might "know" if it is cohabiting with someone by the amount of sex it is getting. It is also known that a woman’s hormonal motivation to have sex is highest on the day of ovulation, and that sex on that day is more likely to result in a girl. Couples who live apart, and therefore probably have intercourse less often, may be more likely to do so when the motivation is highest—resulting in a girl.
    This chain of reasoning, though, provides only what workers in the field call a proximate cause. What is needed for a complete explanation is an ultimate—evolutionary—cause. It is easy to speculate. Perhaps same-sex children are easier for a lone parent to rear. Perhaps parents pass on different kinds of benefits to same-sex offspring and opposite-sex offspring. Perhaps a father helps his son to learn sex-specific skills. Perhaps boys are simply more costly to raise than girls, and would thus overtax the resources of a lone parent. However, a more controversial possibility is that—in a Darwinian sense only —Mr. Battalion is right that two parents are sometimes better than one.
    It is well established, in both humans and other species, that successful males have lots of offspring, while unsuccessful ones have few or none. Females, by contrast, show a smaller range of reproductive output, with most having some offspring, but none having as many as the most successful males. The upshot is that it makes evolutionary sense to have sons when circumstances favour them becoming big, strong, clever and handsome (and therefore attractive to women), but -when they do not, it is better for a woman to have daughters, most of whom will find a mate even in tough times. In the case of humans, circumstances favoring the raising of strong, healthy children could include having two parents around, since humans are unusual among mammals in that fathers are often involved in parental care.
    Of course, even if this evolutionary explanation of Dr. Nordberg’s result does turn out to be correct, it probably does not carry any lessons for the modern world. Such biological patterns would have been established hundreds of thousands—or possibly millions—of years ago. Bringing up children alone in a rich, industrialized society is a rather different proposition from bringing them up in a hunter-gatherer band, and there is no reason to suppose they would he at a disadvantage now. Except, perhaps, that with a surplus of women around, it will he even harder than it is today for a girl to find a suitable husband when she grows up.
What is the main idea of the passage?
选项 A、The proportion of boys being born is dropping.
B、Single mothers are more likely to have daughters.
C、Cohabitation is the decisive factor for newborns’ sex.
D、Both mothers and fathers are involved in parental care.
答案B
解析 主旨题。本文开篇由新生儿出生性别差异的生物学规律引出一个话题：新生儿中男孩对女孩的比例在下降。之后提出Karen Norberg的研究结论：非单身女性生育男孩的几率大于单身女性。之后就此观点展开说明。第五段分析了产生这一现象的生物学原因，接着分析了其他可能的原因。由此可见本文是围绕Dr. Norberg的研究结论展开说明，[B]为主旨，故为答案。[A]是首段提到的问题，引出后文的主旨，排除；[C]是第四段在提出主旨之后，对该观点进行的补充说明，并非主旨；[D]是倒数第二段中出现的细节内容，排除。
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