The number of Mexican immigrants apprehended at the border peaked at 1.6 million in 2000, the Pew Research analysis showed.
There was a similar increase in unauthorized immigrants from Asia. From February 2013 to August 2014, the share of Americans who said the priority should be on better border security when dealing with illegal immigration rose from 25% to 33%.
In 1970, non-Mexican apprehensions totaled just 12,000.
The recent increase in non-Mexican apprehensions is due in part to a surge in unaccompanied Central American child migrants crossing the border without their parents.
By stipulation of the unification agreement, Aden, formerly the capital of South Yemen, functions as the economic centre.
The two components of Yemen underwent strikingly different histories: whereas North Yemen never experienced any period of colonial administration at the hands of a European power, South Yemen was a part of the British Empire from 1839 to 1967.
It is mostly mountainous and generally arid, though there are broad patches with sufficient precipitation to make agriculture successful.