The “precautionary principle” is a rather pretentious late 20th Century term for the common sense old idea that you ought to be more cautious about instituting policies the harder they would be to reverse their effects if they prove bad ideas. For example, importing rabbits into Australia in the 1800s was imprudent because, among other reasons, rabbits breed like rabbits.
The precautionary principle is written into European Union law:
According to the European Commission the precautionary principle may be invoked when a phenomenon, product or process may have a dangerous effect, identified by a scientific and objective evaluation, if this evaluation does not allow the risk to be determined with sufficient certainty.
Recourse to the principle belongs in the general framework of risk analysis (which, besides risk evaluation, includes risk management and risk communication), and more particularly in the context of risk management which corresponds to the decision-making phase.
As we see over and over, if immigrants don’t work out as hoped, it’s not only hard to boot them out of the country, but it’s even politically hard to muster the elite will merely to reduce the number of future immigrants, because the current ones might object to the implicit diss. Instead, as the GOP establishment’s 2013 strategy showed, the easy way to respond to past immigration policy mistakes is to promise to make more of them in the future and to demonize anybody who objects.
Chancellor Merkel’s 2015 whim, for example, would seem like a classic example of when the precautionary principle should have been invoked instead. But this simply doesn’t register in the minds of the kind of people who talk about the precautionary principle.