  The analysis of social networks, in particular those describing face-to-face
interactions between individuals, is complex due to the intertwining of the
topological and temporal aspects. We revisit them both, using public data
recorded by the sociopatterns wearable sensors in some very different
sociological environments, putting particular emphasis on the contact duration
timelines. As well known, the distribution of the contact duration for all the
interactions within a group is broad, with tails that resemble each other, but
not precisely, in different contexts. By separating each interacting pair, we
find that the fluctuations of the contact duration around the mean-interaction
time follow however a very similar pattern. This common robust behavior is
observed on 7 different datasets. It suggests that, although the set of persons
we interact with and the mean-time spent together, depend strongly on the
environment, our tendency to allocate more or less time than usual with a given
individual is invariant, i.e. governed by some rules that lie outside the
social context. Additional data reveal the same fluctuations in a baboon
population. This new metric, which we call the relation *contrast*, can be used
to build and test agent-based models, or as an input for describing long
duration contacts in epidemiological studies.
