In environmental policy, the precautionary principle states that a new product, method, or proposal whose effects are disputed or unknown should not be introduced if it is harmful. The burden of proving that it is harmless falls on its backers—virtually guaranteeing that it won’t be produced. In contrast, a cost-benefit analysis that compares the probability of harm with the expected magnitude of the benefits is a better method.



The methods of the precautionary principle are implicitly applied by many opposing the resettlement of Syrian refugees because they deem any risk of terrorism as too great. The precautionary principle is as improper a standard for determining refugee policy as it is for guiding environmental policy.
Arguments derived from the precautionary principles are often emotionally driven. Sen. Richard Shelby (R-Ala.) made such an appeal when he stated, “We don’t know much about these people. They haven’t really been vetted. They come from an area where there’s a lot of turmoil, a lot of terrorists come from. We don’t need one more terrorist; we got enough right now.”