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  3. A boy or a girl? That is usually the first question asked when a woman gives birth. Remarkably, the answer varies with where the

A boy or a girl? That is usually the first question asked when a woman gives birth. Remarkably, the answer varies with where the

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问题     A boy or a girl? That is usually the first question asked when a woman gives birth. Remarkably, the answer varies with where the mother lives. In rich countries, the chances of its being a boy are about 5 ％ higher than in poor ones. Equally remarkably, that figure has been felling recently. Several theories have been put forward to explain these observations. Some argue that smoking plays a role; others hold that diet may be important. Neither of these ideas has been supported by evidence from large studies. But new research points to a different factor: stress.
    Early studies revealed a shift in women who became pregnant during floods and earthquakes and in time of war. Moreover, a study carried out eight years ago by researchers at the University of Aarhus, in Denmark, revealed that women who suffered the death of a child or spouse from some catastrophic illness around the time they conceived were much more likely to give birth to girls than to boys.
    Taken together, these results suggest that acute stress to a woman at the time of conception shifts the sex ratio towards girls. However, Carsten Obel, a researcher at Aarhus who was not involved in the earlier study, wondered if the same might be true of chronic stress too. In a paper just published in Human Development, he shows that it is.
    Dr. Obel used a set of data collected between 2012 and 2015. During that period 8,719 expectant mothers were asked to fill in questionnaires that inquired, among other things, about their level of stress. Dr. Obel found that the more stressed a mother had been, the less chance she had of having given birth to a boy. Only 47％ of children born to women in the top quartile of stress were males. That compared with 52％ for women in the bottom quartile. Dr. Obel suspects the immediate cause is that male pregnancies are more likely to miscarry in response to stress than female pregnancies are, especially during the first three months. However, that is difficult to prove. More intriguing, though, is the ultimate cause, for he thinks it might be adaptive, rather than pathological.
    That is because the chances are that a daughter who reaches adulthood will find a mate and thus produce grandchildren. A son is a different matter. Healthy, strapping sons are likely to produce lots of grandchildren, by several women—or would have done in the hunter-gatherer societies in which most human evolution took place. Weak ones would be marginalized and maybe even killed in the cut and thrust of male competition. If a mother’s stress adversely affects the development of her fetus then selectively aborting boys, rather than wasting time and resources on bringing them to term, would make evolutionary sense.
    That, in turn, would explain why women in rich countries, who are less likely to suffer from hunger and disease, are more likely to give birth to sons. That this likelihood is, nevertheless, falling suggests that rich women’s lives may be more stressful than they used to be.
The ratio of giving birth to a boy is falling in rich countries because________.
选项 A、wars exerted huge negative impact
B、women are facing greater pressure than past
C、women are under new pressure now which they seldom faced in the past
D、male pregnancies are more easily to miscarry
答案B
解析 本题关键词是birth to a boy和rich countries，问题是：富裕国家男孩的出生率下降的原因是什么?本题要定位全文。第六段最后一句话指出，富裕国家的女性更容易生男孩，这种概率的下降说明富裕的女性目前所受的压力可能要比以前大(more stressful than they used to be)，因此选项B与原文属于相同含义，是正确答案。选项A属于以偏概全，第二段提到洪水、地震和战争期间怀孕的女性生育男女的比例会发生变化，归根究底是压力造成男孩出生率的下降，该选项表述不正确。选项C属于主观推导，因为第六段只是说富裕的女性目前所受的压力可能要比以前大，但并不是有新的压力(new pressure)出现。选项D属于就事论事，因为文章第四段第六句提到在有压力的情况下，怀男孩的女性比怀女孩的女性更容易流产，选项D忽略了压力这个前提。第二段：用研究来证明突发压力对于婴儿性别的影响。第三段：卡斯滕.奥贝尔想知道持续的压力是否也会影响婴儿性别。第四段：研究表明慢性压力的确会影响婴儿性别，但奥贝尔认为这是适应能力的体现。第五段：从进化论的角度解释压力导致女婴增加的说法是合理的。第六段：发达国家压力小，女性生男孩的几率大，而富裕的女性压力大，则容易生女孩。
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