  Cognition has been found to constrain several aspects of human behaviour,
such as the number of friends and the number of favourite places a person keeps
stable over time. This limitation has been empirically defined in the physical
and social spaces. But do people exhibit similar constraints in the digital
space? We address this question through the analysis of pseudonymised mobility
and mobile application (app) usage data of 400,000 individuals in a European
country for six months. Despite the enormous heterogeneity of apps usage, we
find that individuals exhibit a conserved capacity that limits the number of
applications they regularly use. Moreover, we find that this capacity steadily
decreases with age, as does the capacity in the physical space but with more
complex dynamics. Even though people might have the same capacity, applications
get added and removed over time. In this respect, we identify two profiles of
individuals: app keepers and explorers, which differ in their stable (keepers)
vs exploratory (explorers) behaviour regarding their use of mobile
applications. Finally, we show that the capacity of applications predicts
mobility capacity and vice-versa. By contrast, the behaviour of keepers and
explorers may considerably vary across the two domains. Our empirical findings
provide an intriguing picture linking human behaviour in the physical and
digital worlds which bridges research studies from Computer Science, Social
Physics and Computational Social Sciences.
