  Elections constitute a paradigm of decision-making problems that have puzzled
experts of different disciplines for decades. We study two decision-making
problems, where groups make decisions that impact only themselves as a group.
In both studied cases, participation in local elections and the number of
democratic representatives at different scales (from local to national), we
observe a universal scaling with the constituency size. These results may be
interpreted as constituencies having a hierarchical structure, where each group
of $N$ agents, at each level of the hierarchy, is divided in about $N^{\delta}$
subgroups with $\delta \approx 1/3$. Following this interpretation, we propose
a phenomenological model of vote participation where abstention is related to
the perceived link of an agent to the rest of the constituency and which
reproduces quantitatively the observed data.
