We analyze the secular decline in interstate migration in the United States between 1991 and 2011. Gross flows of people across states are about 10 times larger than net flows, yet have declined by around 50 percent over the past 20 years. We argue that the fall in migration is due to a decline in the geographic specificity of returns to occupations ["it matters less where your job is than what your job is"], together with an increase in workers' ability to learn about other locations before moving there, through information technology and inexpensive travel. These explanations find support in micro data on the distribution of earnings and occupations across space and on rates of repeat migration. Other explanations, including compositional changes, regional changes, and the rise in real incomes, do not fit the data.This is how the authors define gross migration flows:
". . . the fraction of U.S. residents at least 1 year old who lived in a different state one year ago . . ."And net migration flows:
The numerator of the net migration rate is one-half of the sum of absolute values of inflows minus outflows in each state. This number is the minimum number of moves that would have to be prevented to set net migration to zero in every state. The denominator of the rate between years t and t + 1 is the U. S. population at t minus deaths between t and t + 1 [not very intuitive].The Wikipedia definition of "net migration rate" is here.