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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.
Women in the hunter-gatherer societies are more likely to give birth to daughters because________.
选项 A、they agree that giving birth to daughters is beneficial in the evolutionary sense
B、sons are likely to produce lots of grandchildren with several women
C、they think it is a better practice for a daughter to produce grandchildren with only one mate
D、they think bringing sons to term is wasting time and resources
答案D
解析 本题关键词是women、hunter—gatherer societies和daughters，问题是：在原始狩猎社会中，女性生女孩的概率更大的原因是什么?可以定位到第五段。根据文章第五段，在原始狩猎社会中，健康、魁梧的男性更有可能与几位女性生育很多后代，而弱小的男性则会被边缘化，甚至有可能在男性的竞争性打斗中被杀死。如果母亲受到的压力对胎儿生长有负面影响，她更情愿将男婴流产，也不愿意花费时间和资源后将男婴生出来，却面临被淘汰的压力。所以在原始狩猎社会中，女孩出生率较高的原因是因为女性生育时面临的这种压力，因此选项D属于全面概括，为正确答案。选项A属于主观推导，因为题干的前提是在原始狩猎社会(in the hunter—gatherer societies)，原始人不会从进化的(evolutionary)角度去思考问题。选项B属于答非所问，选项B的说法只是一个现象，并不是题干中间的原因。选项C属于无中生有，原文并未提及。
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