So Donald Trump Jr. tweeted a metaphor about poisoned Skittles and Syrian refugees. It's drawn directly from a Nazi children's book which uses the same metaphor for Jews, who are the "poisonous mushrooms" you must not gather in the forest. It's worth pointing out that the author was hanged at Nuremberg (same article).
We have some actual data on this. Since 9/11, 745,000 Syrian refugees came to the US (until late 2015), and there have been two (2!) terrorism related arrests among them (both for financing terrorism in other countries). That's 2/745,000 or 0.27 per 100,000 Syrian refugees vetted and brought to the US. About 1/4 of a terrorist per 100,000 refugees admitted.
How much does that raise our risk? The pool of terrorist attacks in the US is small. Let's start with violent felonies, instead.
As of 2012, there were just over 721,000 convicted violent felons incarcerated in the US. That's 721,000 out of roughly 330 million, or about 240 per 100,000 Americans (including men, women, and children). So there are 1000 times as many violent felons in the US as there are terrorists in vetted Syrian refugees. Adding those refugees raised our risk by 2 in 721,000. That's a number so small it's not worth doing the math.
How about actual acts of terrorism within the US? There have been 491 individuals designated as having "launched extremist attacks" in the US since 2001. Of those, 397 were US citizens, 39 were legal residents, 10 refugees, and 8 held visas, while 37 had unknown or "other" citizenship status. According to the fact checking article just referenced:
Individuals were counted as extremists if they have been indicted on or convicted of terrorism-related crimes, or had been killed and “widely and credibly reported” as having extremist motives, according to New America’s website.
Which is a MUCH higher bar than funding terrorism elsewhere.
397 American citizens having launched an extremist attack out of 330 million is about 0.12 per 100,000. Only about half as many as Syrian refugees have funded terrorism overseas -- about twice as many.
To be clear, twice as many vetted Syrian refugees have been convicted of funding terrorism overseas as American citizens have launched actual terrorist attacks against the United States.
Of the 28 deadly homegrown terrorist attacks in the US since 9/11, 18 have been made by violent right-wing extremists, versus 10 violent (homegrown) Islamists. Adding .4% to the pool of violent Islamists would up that total from 10 to 10.4, or maybe one, making the ratio 11 to 18.
So adding all those Syrian refugees might, possibly, have added one deadly terrorist attack in the last 15 years, except that the Syrian refugee "terrorists" were funding terrorism overseas, not making attacks here.
Syrian refugees are not three in a bowlful of Skittles. They are 1 in 400,000. Since there are about 400 Skittles per pound (Thank you, Walmart!), that means one dangerous Skittle per half ton.
I'll take those odds. Remember that Steve Jobs was the son of a Syrian political refugee.