  We present a new, general constraint which, in principle, determines the
superconformal $U(1)_R$ symmetry of 4d $\N =1$ SCFTs, and also 3d $\N =2$
SCFTs. Among all possibilities, the superconformal $U(1)_R$ is that which
minimizes the coefficient, $\tau_{RR}$, of its two-point function.
Equivalently, the superconformal $U(1)_R$ is the unique one with vanishing
two-point function with every non-R flavor symmetry. For 4d $\N =1$ SCFTs,
$\tau_{RR}$ minimization gives an alternative to a-maximization. $\tau_{RR}$
minimization also applies in 3d, where no condition for determining the
superconformal $U(1)_R$ had been previously known. Unfortunately, this
constraint seems impractical to implement for interacting field theories. But
it can be readily implemented in the AdS geometry for SCFTs with AdS duals.
