Trust transitivity in social networks

TL;DR

This paper analyzes how trust propagates in social networks using a simple transitivity metric, revealing conditions for global trust spread and comparing community- and authority-centered trust scenarios in the PGP web of trust.

Contribution

It provides an analytical framework for trust percolation in networks and compares different trust distribution models in real-world systems.

Findings

A non-zero fraction of absolute trust is necessary for global trust propagation.

Trust percolation exhibits a discontinuous transition at a critical trust level.

Authority-centered trust schemes are more efficient but favor fringe nodes.

Abstract

Non-centralized recommendation-based decision making is a central feature of several social and technological processes, such as market dynamics, peer-to-peer file-sharing and the web of trust of digital certification. We investigate the properties of trust propagation on networks, based on a simple metric of trust transitivity. We investigate analytically the percolation properties of trust transitivity in random networks with arbitrary degree distribution, and compare with numerical realizations. We find that the existence of a non-zero fraction of absolute trust (i.e. entirely confident trust) is a requirement for the viability of global trust propagation in large systems: The average pair-wise trust is marked by a discontinuous transition at a specific fraction of absolute trust, below which it vanishes. Furthermore, we perform an extensive analysis of the Pretty Good Privacy (PGP) web of trust, in view of the concepts introduced. We compare different scenarios of trust distribution: community- and authority-centered. We find that these scenarios lead to sharply different patterns of trust propagation, due to the segregation of authority hubs and densely-connected communities. While the authority-centered scenario is more efficient, and leads to higher average trust values, it favours weakly-connected "fringe" nodes, which are directly trusted by authorities. The community-centered scheme, on the other hand, favours nodes with intermediate degrees, in detriment of the authorities and its "fringe" peers.