The independent Migration Advisory Committee came up with the earnings threshold when asked by the government to calculate the minimum needed to avoid recourse to public funds. But £18,600 is half again as much as someone would make working 40 hours a week, 52 weeks a year, at the minimum wage. According to the Migration Observatory at Oxford University, 47% of British citizens in employment would not qualify to bring in a family member, nor would 58% of people 20-30 years old, or 61% of women of any age.
http://www.economist.com/node/21556926
