  Social life clusters into groups held together by ties that also transmit
information. When collective problems occur, group members use their ties to
discuss what to do and to establish an agreement, to be reached quick enough to
prevent discounting the value of the group decision. The speed at which a group
reaches consensus can be predicted by the algebraic connectivity of the
network, which also imposes a lower bound on the group's cohesion. This
specific measure of connectivity is put to the test by re-using experimental
data, which confirm the prediction.
