Estimation of a symmetric density

Kraft, Lepage, and van Eeden (1985) have suggested using a symmetrized version of the kernel estimator when the true density f of the observation is known to be symmetric around a possibly unknown point θ. The effect of this symmetrization device depends on the smoothness of f * f(x) = f f(x+t)f(t) dt at zero. We show that if θ has to be estimated and if f is not absolutely continuous, symmetrization may deteriorate the estimate.