The more difficult crossing the border becomes, the more opportunities there are for the human traffickers known as coyotes, who promise to steer those heading north to safe houses or cars driven by accomplices or relatives, at prices ranging as high as $2,000 a head.
This implies that comparisons of the possible influence of sociodemographic factors are more difficult cross-culturally.
"Anytime you make it more difficult to cross, you empower cartel and smugglers and organized criminals who control the crossing points and the border," said Bier. .
Its fighters may find it more difficult to cross the Syrian-Turkish border, though even this is uncertain.
The eurosceptic politician said the UK should step up intrusive checks on cars and lorries to make it more difficult to cross borders.
More broadly, the spectacular increase in American inequality has made the gap between the rich and the middle class wider, and hence more difficult to cross, than it was in the past.
Every morning, hundreds of Iraqis pile into buses and taxis from east Amman heading back home, though the flow is less in recent days, as it has become more difficult to cross the border.
"If we get too close to the end of our season, then it is very difficult for the teams because the intensity of matches, quarter-finals of cup competitions, things like that, just makes it more and more difficult to cross the Atlantic.
Mr Borchashvili says this is partly because it is far more difficult to cross the borders of the North Caucasus than it used to be, but also because the young Chechens believe it is incumbent on them to help "protect oppressed people" in Syria.
Each of us had our neighborhood, and it was much more difficult to cross lines.
The GDR decided to upgrade the fortifications in the late 1960s to establish a "modern frontier" that would be far more difficult to cross.
