Illegal reorientation is especially problematic when government restrictions are placed on family reunification.
While imposing visa restrictions can reduce overall migration, it may push more would-be migrants into unauthorised channels, according to a study.
Researchers from the University College London in the UK investigated how individuals are likely to move from one country to another based on varying levels of restriction.
In particular, they looked at student and high-skilled visas, low-skilled and family visas.
The study, published in the journal PNAS, found that restricting students and those eligible for high-skilled visas does little to change the overall volume or composition of incoming migrants.
Whereas restricting family and low-skilled visas appears to reduce overall migration, it also diverts a significant portion of aspiring migrants to unauthorised channels.
Illegal reorientation is especially problematic when government restrictions are placed on family reunification, where roughly a quarter of all those who would have migrated legally would, instead, move abroad through illegal channels.
"We show that even minimal visa requirements can significantly reduce immigration, but this comes at the cost of reorienting aspiring migrants towards unauthorised channels," said Miranda Simon from University College London.
"The largest reorientation towards unauthorised channels happens when the family route is closed, because it is the most easily accessible out of those considered," said Simon.
"When restricting immigration policy, governments need to consider that they are also reducing aspiring migrants' already limited options for legal migration," Simon said.
The study found that under a baseline policy scenario in which anyone could migrate as long as they met minimal visa eligibility requirements, only 44 per cent of aspiring migrants moved abroad through legal channels.
When further restrictions are added, such as limiting work allowances for student migrants or increasing sponsorship burdens on families abroad, legal migration becomes increasingly difficult.
Restricting low-skilled worker or family migration reduced immigration by 21 and 32 per cent respectively from baseline levels, but also increased unauthorised immigration by 14 per cent and 24 per cent, respectively.
Results also show that enforcement of unauthorised migration is generally not an efficient solution as more than 80 per cent of unauthorised migrants would need to be apprehended to offset the effects of legal restrictions.
Researchers used a data-driven, agent-based computational model (ABM) to examine migration for one corridor - ie individuals moving from one origin country to one destination country.
They examined the effects of immigration policy restrictions on common channels such as family reunification, student, low-skilled work and high-skilled work across major destination countries.