A simple evolution equation is derived for b(t), the ratio of gravitational correlation energy to kinetic energy in the expanding universe, which gives the observed asymptotic value of 0.7 and also predicts the behavior of the ratio at earlier times. The equation is obtained by combining the cosmic energy equation with the BBGKY clustering hierarchy. The relationship of the solutions of b(t) to the thermodynamic description of gravitational clustering in the expanding universe is briefly discussed. 13 references.