It is generally believed (=thought) that the chief reason for the increase in population in developed countries is not so much the rise in birth rates as the decline in death rates as a result of the improvement in medical care.

• 普遍认为，在发达国家人口增长的主要原因与其说是出生率的上升，还不如说是由于医疗保健的改善的使死亡率下降了。