See also: swamped and moderate versus radical open borders.

According to its Wikipedia page, the precautionary principle states that:

if an action or policy has a suspected risk of causing harm to the public or to the environment, in the absence of scientific consensus that the action or policy is harmful, the burden of proof that it is not harmful falls on those taking the action.

Some opponents of immigration argue that given the uncertainties surrounding immigration, the precautionary principle dictates restrictionist policies.

Others who are generally supportive of immigration still consider the precautionary principle as a justification for allowing only modest and gradual increases in immigration and reject a policy of open borders (see also moderate versus radical open borders).

Here are some examples of arguments based on the precautionary principle:

• Peter Brimelow writes in Alien Nation: Common Sense about America’s Immigration Disaster, quotes from a letter written by a subscriber to National Review:
  

  Readers who find themselves bogged down in the pros and cons of [Third World] immigration can simplify the issue by asking the same question airplane pilots and sailors pose before setting off in threatening weather, “What if?”

  What if Julian Simon, Ben Wattenberg and Bob Bartley are wrong? Are we prepared for any or all of the possible consequences: public assistance programs watered down or driven out completely, our environ-
  ment overwhelmed, massive poverty, a shrinking percentage of our population in good jobs, a splintered society in which ethnic strife is as common as a rainy day, leading to that time in the next century when the U.S. goes totalitarian because the nation is no longer governable as a republic with four huge ethnic minorities (one of which is white)? . . .

  Now, what if Peter Brimelow and George Borjas are wrong? Can we handle a labor shortage and a reduction in the spread of fast food restaurants? We can solve the labor problem by going to our southern bor-
  der and whispering, “we can use a few workers. ” As for the restaurants, here’s a vote for more home cooked meals.

  — Tev Laudeman, Louisville, Ky.

See also:

• Swamped: Discussion of concerns about the large-scale immigration that might happen if borders were suddenly opened.
• Moderate versus radical open borders.

Responses by open borders advocates

In his piece Why Should We Restrict Immigration? (PDF, 20 pages, ungated), part of Cato Journal Winter 2012, Bryan Caplan addresses the precautionary principle by writing:

If you embrace something like the Precautionary Principle (Sunstein 2005), this is a powerful objection to immediate open borders. The society we have works extremely well by world and historic standards. If you live in the First World, you’re doing fine. Why take chances?

From an amoral, risk-averse point of view, there is no good response to this objection. But if you take the moral presumption in favor of free migration seriously, this is a weak argument indeed. Immigration restrictions are not a minor inconvenience we impose on the rest of the world for our peace of mind. Immigration restrictions literally ruin many millions of lives—forcibly denying people the opportunity to do business with their best customers. “We’re trapping millions in Third World misery because we know that free migration has very bad consequences” arguably overcomes the presumption in favor of open borders. “We’re trapping millions in Third World misery because there’s a small chance that free migration has very bad consequences” does not. Think of the moral progress that the Precautionary Principle would have precluded: until a society tried freedom of religion or the abolition of slavery, no one could be sure the experiment wouldn’t end in disaster.

In any case, the Precautionary Principle lends no support to the status quo. Existing research confirms that moderate liberalization of immigration has excellent overall consequences. If the “out of sample” problem bothers you, the obvious solution is to expand the sample gradually. Step one: liberalize slightly more than any other country. Step two: see what happens. Step three: in the absence of very bad consequences, liberalize a little more and return to step two.