  The four-fifths law for third-order longitudinal moments is examined, by the
use of direct numerical simulation data on three-dimensional forced
incompressible magnetohydrodynamic (MHD) turbulence without a uniformly imposed
magnetic field in a periodic box. The magnetic Prandtl number is set to one,
and the number of grid points is $512^3$. A generalized
K\'arm\'an-Howarth-Kolmogorov equation for second-order velocity moments in
isotropic MHD turbulence is extended to anisotropic MHD turbulence by means of
a spherical average over the direction of $\textbf{r}$. Here, $\textbf{r}$ is a
separation vector. The viscous, forcing, anisotropy and nonstationary terms in
the generalized equation are quantified. It is found that the influence of the
anisotropic terms on the four-fifths law is negligible at small scales,
compared to that of the viscous term. However, the influence of the directional
anisotropy, which is measured by the departure of the third-order moments in a
particular direction of $\textbf{r}$ from the spherically averaged ones, on the
four-fifths law is suggested to be substantial, at least in the case studied
here.
