Current data from the Planck satellite and the BICEP2 telescope favor , at around the 2 \sigma level , negative running of the spectral index of curvature perturbations from inflation .
We show that for negative running \alpha < 0 , the curvature perturbation amplitude has a maximum on scales larger than our current horizon size .
A condition for the absence of eternal inflation is that the curvature perturbation amplitude always remain below unity on superhorizon scales .
For current bounds on n _ { S } from Planck , this corresponds to an upper bound of the running \alpha < -4 \times 10 ^ { -5 } , so that even tiny running of the scalar spectral index is sufficient to prevent eternal inflation from occurring , as long as the running remains negative on scales outside the horizon .
In single-field inflation models , negative running is associated with a finite duration of inflation : we show that eternal inflation may not occur even in cases where inflation lasts as long as 10 ^ { 4 } e-folds .