The study by our bd4d team built on the Social Connectedness Index concept introduced by Michael Bailey (the team lead for economics research at Facebook) and others.
For most of human history, people have moved. It is only in the relatively short window after the establishment of the Westphalian state, especially after the collapse of the empires after the Second World War, that these movements have been constrained. The logic of globalization is based on the mobility of factors of production. Some economists like Paul Collier have chosen to ignore the need for labor to be mobile too. Those who see the technology glass as half-empty have seen the strong surveillance capabilities of the state as putting an end to movement of people across borders.
As a research organization we like data. We worry about the best indicators, for anything. For the longest time, per capita GDP has been the simplest, least-imperfect indicator of prosperity. It has many shortcomings, but the alternatives have more. The newest run at it zooms in on happiness.