他们的迁徙可能是人口增加引起的，这一人口增长是由人们逃离撒哈拉沙漠的干燥或干涸的迁移造成的。

Their migration may have been set in motion by an increase in population caused by a movement of peoples fleeing the desiccation, or drying up, of the Sahara.

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斯维姆说，“从事一类对环境有益的事能使一人偏向做相似的行为，抑制别样行为，或者甚至能增加对环境有害的行为。”

"Engaging in one type of environmentally friendly behavior can predispose one to engage in similar behaviors, inhibit other behaviors, or even increase environmentally harmful behaviors," said Swim.

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这一人口变化带来的结果是，60岁以上的人在数量和比例上持续增加。

This demographic change has resulted in increasing Numbers and proportions of people who are over 60.

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