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TEXT TEN A boy or a girl? That is usually the first question asked when awoman 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 inpoor ones. Equally remarkably, that figure has been falling recently. Severaltheories have been put forward to explain these observations. Some argue thatsmoking plays a role, others that diet may be important. Neither of these ideashas been supported by evidence from large studies. But new research points to adifferent factor: stress. Strange as it might seem, the terrorist attacks of September 11th2001 shed light on the enigma. Studies noting the sex of babies conceived inNew York during the week of the attacks found a drop in the ratio of males tofemales. That is consistent with earlier studies, which revealed a similarshift in women who became pregnant during floods and earthquakes and in time ofwar. Moreover, a study carried out eight years ago by researchers at theUniversity of Aarhus, in Denmark, revealed that women who suffered the death ofa child or spouse from some catastrophic illness around the time they conceivedwere much more likely to give birth to girls than to boys. Taken together, these results suggest that acute stress to a womanat the time of conception shifts the sex ratio towards girls. However, CarstenObel, 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 justpublished in Human Development, he shows that it is. | 
