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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  32
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.
Which of the following is NOT mentioned as a possible reason?
选项 A、It’s easier for a single mother to provide for a daughter.
B、A lone parent takes fewer pains to bring up a same-sex child.
C、A lone parent may be more considerate of a same-sex child.
D、A single parent teaches a same-sex child some particular skills.
答案C
解析 细节题。根据选项中的lone parent定位至第六段。作者从第四句开始分析单亲家庭的孩子性别趋向与大人一致的可能原因，这里只有单亲父母更能体谅同性别子女这个原因没有提及，故[C]为答案。第七句指出“Perhaps boys are simply more costly to raise than girls．and would thus overtax the resources of a lone parent．”，[A]与本句相符；[B]是对第四句“Perhaps same sex children are easier for a lone parent to rear．”的解释；[D]可以概括第六句“Perhaps a father helps his son to learn sex-specific skills．”，均符合文意。
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