I can see these being people who are going to be considered as high value - they must have to show means of being able to support themselves though mustn't they as you have to pay Social security contributions to be eligible for benefits?
I looked at some history - as the OP
was asking for that sort of info and its quite interesting to see how the changes affected the figures
The first immigrants were mostly British and for obvious reasons the numbers coming in were low. The laws were introduced when 'people of colour' began to flood in. In the 1790 Act only 'Free White People' were allowed into the country - they did bring in indentured servants (slaves black or white) as presumably they were considered to be possessions and not people
In the 1860's Blacks were allowed entry but it wasnt until the 1950's that Asians were allowed following the Page Act of 1875 that prevented Asians entry to 'prevent cheap forced labour, prostitution and people fleeing from criminal acts'
In 1924 a Law was passed to restrict the entry of southern & eastern europeans - mostly aimed at Jews, Italians and Slavs
After WW2 Most refugees were barred from entry
In the 1930 depression era more people left the US than entered
After the ethnic quota rule was removed in 1965 the numbers of immigrants quadrupled - 9.6m in 1970 to 38m in 2007
The highest numbers of immigrations now come from Mexico, India, China & Phillipines
Two thirds of immigrants (legal) are now family members of the original immigrant - and this is where you can see the problem spiralling exactly as it has in the UK - because in 2009 66% of immigrants came in this way against only 13% who came in through employment skills.