  The peer-to-peer (P2P) economy relies on establishing trust in distributed
networked systems, where the reliability of a user is assessed through digital
peer-review processes that aggregate ratings into reputation scores. Here we
present evidence of a network effect which biases digital reputation, revealing
that P2P networks display exceedingly high levels of reciprocity. In fact,
these are much higher than those compatible with a null assumption that
preserves the empirically observed level of agreement between all pairs of
nodes, and rather close to the highest levels structurally compatible with the
networks' reputation landscape. This indicates that the crowdsourcing process
underpinning digital reputation can be significantly distorted by the attempt
of users to mutually boost reputation, or to retaliate, through the exchange of
ratings. We uncover that the least active users are predominantly responsible
for such reciprocity-induced bias, and that this fact can be exploited to
obtain more reliable reputation estimates. Our findings are robust across
different P2P platforms, including both cases where ratings are used to vote on
the content produced by users and to vote on user profiles.
