  People interact face-to-face on a frequent basis if (i) they live nearby and
(ii) make the choice to meet. The first constitutes an availability of social
ties; the second a propensity to interact with those ties. Despite being
distinct social processes, most large-scale human interaction studies overlook
these separate influences. Here, we study trends of interaction, availability,
and propensity across US cities for a critical, abundant, and understudied type
of social tie: extended family that live locally in separate households. We
observe a systematic decline in interactions as a function of city population,
which we attribute to decreased non-coresident local family availability. In
contrast, interaction propensity and duration are either independent of or
increase with city population. The large-scale patterns of availability and
interaction propensity we discover, derived from analyzing the American Time
Use Survey and Pew Social Trends Survey data, unveil previously-unknown effects
on several social processes such as the effectiveness of pandemic-related
social interventions, drivers affecting residential choice, and the ability of
kin to provide care to family.
