  In the systems showing critical behavior, various response functions have a
singularity at the critical point. Therefore, as the driving field is tuned
towards its critical value, the response functions change drastically,
typically diverging with universal critical exponents. In this work, we
quantify the inequality of response functions with measures traditionally used
in economics, namely by constructing a Lorenz curve and calculating the
corresponding Gini index. The scaling of such a response function, when written
in terms of the Gini index, shows singularity at a point that is at least as
universal as the corresponding critical exponent. The critical scaling,
therefore, becomes a single parameter fit, which is a considerable
simplification from the usual form where the critical point and critical
exponents are independent. We also show that another measure of inequality, the
Kolkata index, crosses the Gini index at a point just prior to the critical
point. Therefore, monitoring these two inequality indices for a system where
the critical point is not known, can produce a precursory signal for the
imminent criticality. This could be useful in many systems, including that in
condensed matter, bio- and geophysics to atmospheric physics. The generality
and numerical validity of the calculations are shown with the Monte Carlo
simulations of the two dimensional Ising model, site percolation on square
lattice and the fiber bundle model of fracture.
