> Evidence actually points in the opposite direction:
I was speaking about the United States: can you find a study that somehow documents illegal immigrant (by definition: undocumented) persons' productivity?
Folks brought over on legal immigration visas likely do make more money (and contribute more) than the average American: that's why we have these programs for 'exceptional' individuals. Nobody is going through the effort (nor can they afford the costs) of obtaining lawful visas for construction labor or meat processing staff.