It seems to be the case that all the terrorists identified in the Paris massacre were Europeans. Born and raised there. And it also seems to be the case that many of the young people joining "Isis" weren't raised in extremist households and most of those from "the West" are signing up for the identity, purpose, thrill and adventure of taking part in what they see as a dramatically historic movement and cause.
In fact, it seems, at least to me, that there is a lot more in common between the homegrown terrorists we have in the USA (like the Oklahoma bomber and the school shooters, etc.), who have caused more deaths than any coming from outside the country (including 9/11) and the killers in Paris.
Closing our borders to Syrian refugees fleeing the death and destruction in their homeland in hopes of preventing tragedies like the Paris mass killings will have no impact on the mass murders being committed in the USA on a regular basis.