In the years following the terrorist attacks of September 11, 2001, for example, legitimate security concerns led to enormous student-visa delays and bureaucratic hassles for foreigners aspiring to study in the US. Student numbers have since rebounded, despite intermittent problems, but there remain severe limits on work and residency visas, which should serve as an enticement for the best and brightest to study in the US.
同时，在人员的国际流动方面还存在着另外一些障碍，虽然不都是以保护主义的形式存在，但都起到了阻止学生奔赴全世界各大学府的作用。比如说“9·11”事件爆发之后，安全方面的考虑就导致大量学生签证延迟并遭到政府部门的百般刁难。虽然后来学生数量在磕磕碰碰中有所反弹，但在工作和居留签证方面依然卡得很死，而这两项签证却恰恰是吸引那些最优秀学生留学美国的最大诱惑。