The effect of immigration on things like crime rate, wages, and unemployment are frequently discussed, but there's something that often isn't which I think is one of the worst consequences: the day-to-day constant breakdown of previously unspoken social rules. You can go your whole life without being stabbed by Ahmed, but if you have a 3rd world immigrant problem this is completely inescapable and constant in a way that I think does serious damage to the payche. Little things like waiting your turn, walking on the right (or left, as the country goes) side of hallways, treating communal objects/tools/spaces with respect, etc.; things which were once normal no longer are, and when you have say 10% of your population disregarding the rules completely, a good chunk of the other 90% will eventually give and stop bothering. Why act politely when no one else does, when the other guy might just take advantage of it without offering anything back to society?