  Structural inequalities persist in society, conferring systematic advantages
to one group of people, for example, by giving them substantially more
influence and opportunities than others. Using bibliometric data about authors
of scientific publications from six different disciplines, we first present
evidence for the existence of two types of citation inequalities. First, female
authors, who represent a minority in each discipline, receive less recognition
for their work relative to male authors; second, authors affiliated with
top-ranked institutions, who are also a minority in each discipline, receive
substantially more recognition compared to other authors. We then present a
dynamic model of the growth of directed citation networks and show that such
systematic disparities in citations can arise from individual preferences to
cite authors in the same group (homophily) or the other group (heterophily),
highly cited or active authors (preferential attachment), as well as the size
of the group and how frequently new authors join. We analyze the model
theoretically and show that its predictions align well with real-world
observations. Our theoretical and empirical analysis sheds light on potential
strategies to mitigate structural inequalities in science. In particular, we
find that merely making group sizes equal does little to narrow the
disparities. Instead, reducing the homophily of each group, frequently adding
new authors to a research field while providing them an accessible platform
among existing, established authors, together with balanced group sizes can
have the largest impact on reducing inequality. Our work highlights additional
complexities of mitigating structural disparities stemming from asymmetric
relations (e.g., directed citations) compared to symmetric relations (e.g.,
collaborations).
