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. Senator Shelby (R-AL) 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.”
Senator Shelby is correct that we don’t need another terrorist, but he didn’t explain that the risk of a terrorist coming through the refugee system is low.
3,252,493 refugees were admitted to the United States from 1975-2015. During that time period, 20 of those individuals attempted to carry out a terrorist attack or succeeded in doing so inside of the United States. That is a single terrorist for every 162,625 refugees admitted or one every two years since 1975.
Although there were only 20 refugee terrorists admitted since 1975, they have only succeeded in murdering three Americans. Each one of those murders is a tragedy but the chance that an American would be successfully killed by a refugee terrorist was one in 3.6 billion a year. Each year an American had a 0.000000028 percent chance of being murdered by a refugee terrorist each year (for those with poor eyesight, that’s seven zeros to the right of the decimal point). That’s a small risk.