http://www.cesifo-group.de/ifoHome/publications/working-papers/CESifoWP/CESifoWPdetails?wp_num=6441&CESifoWP.search=+
https://www.gainesvilletimes.com/opinion/letter-why-mexico-border-wall-probably-waste-time-money/
http://www.cesifo-group.de/ifoHome/publications/working-papers/CESifoWP/CESifoWPdetails?wp_num=6452&CESifoWP.search=+