This result shows that our estimates are not significantly affected by the strength of friendships or the number of Facebook friends that people have.

Finally, measures of civic engagement do not use data on networks at all and are instead based purely on individual or community-level characteristics (Supplementary Table 6).

(1) cross-type connectedness, which is the extent to which different types of people (for example, high income versus low income) are friends with each other15,30,31,32; (2) network cohesiveness, which is the degree to which friendship networks are clustered into cliques and whether friendships tend to be supported by mutual friends33; and (3) civic engagement, which we measure using indices of trust or participation in civic organizations34,35.

As a result, our analysis does not shed light on the effects of online social networks themselves.

More recent studies have used large-scale mobile phone data to measure ‘experienced segregation’1,17,18,19,20,21 but do not directly observe social interactions between different types of people, a distinction that we show is empirically important.

If children with low-SES parents were to grow up in counties with economic connectedness comparable to that of the average child with high-SES parents, their incomes in adulthood would increase by 20% on average.

SES

Social capital—the strength of an individual’s social network and community—has been identified as a potential determinant of outcomes ranging from education to health1,2,3,4,5,6,7,8.

We measure and analyse three types of social capital by ZIP (postal) code in the United States: (1) connectedness between different types of people, such as those with low versus high socioeconomic status (SES); (2) social cohesion, such as the extent of cliques in friendship networks; and (3) civic engagement, such as rates of volunteering.