  We introduce a model-independent approximation for the branching ratio of
Hawkes self-exciting point processes. Our estimator requires knowing only the
mean and variance of the event count in a sufficiently large time window,
statistics that are readily obtained from empirical data. The method we propose
greatly simplifies the estimation of the Hawkes branching ratio, recently
proposed as a proxy for market endogeneity and formerly estimated using
numerical likelihood maximisation. We employ our new method to support recent
theoretical and experimental results indicating that the best fitting Hawkes
model to describe S&P futures price changes is in fact critical (now and in the
recent past) in light of the long memory of financial market activity.
