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　　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 falling recently. Several theories have been put forward to explain these observations. Some argue that smoking plays a role, others 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.

　　Strange as it might seem, the terrorist attacks of September 11th 2001 shed light on the enigma. Studies noting the sex of babies conceived in New York during the week of the attacks found a drop in the ratio of males to females. That is consistent with earlier studies, which revealed a similar 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 1989 and 1992. 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 marginalised 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.

　　1. The author begins the passage by_____

　　A. presenting an argumentation.

　　B. explaining a phenomenon.

　　C. raising a question.

　　D. making a comparison

　　2. The ratio of giving birth to a boy is falling in rich countries because_____

　　A. the terrorist attacks of September 11th 2001 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.

　　3. Which of the following can explain Dr Obel’s opinion that the ultimat cause is adaptive rather than pathological?

　　A. 47% of children born to women in the top quartile of stress were males while 52% in the bottom quartile.

　　B. Women in rich countries are more likely to give birth to boys.

　　C. Women selectively abort boys rather than waste time and resources on bringing them to term for fear of male competition.

　　D. Women who suffer from calamity in conception are more likely to give birth to girls.

　　4. Women in the hunter-gatherer societies are more likely to give birth to daughters because_____

　　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.

　　5.From this passage, we may draw a conclusion that_____

　　A. acute stress is more likely to cause women to choose aborting boys than chronic stress.

　　B. stress to a woman at the time of conception, whether acute or chronic, will shift the sex ratio towards girls.

　　C. more girls will be born in the future because today’s women, in both rich and poor countries, suffer from increasing pressure.

　　D. chronic stress is more decisive in influencing the women’s pregnancies.

　　[答案]1. A 2. B 3. C 4. D 5. B

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