  The scaling of nuclear spin-dependent parity violating effects with
increasing nuclear charge $Z$ is discussed in two series of isovalent
open-shell diatomic molecules. The parameter $W_\mathrm{a}$ characterising the
strength of parity violation in diatomic molecules is calculated in the
framework of the zeroth-order regular approximation (ZORA) and found to be in
good agreement with the $R(Z) Z^k$ scaling law derived for atoms in which
$R(Z)$ represents a relativistic enhancement factor. The influence of electron
correlation is studied on the molecular level, with spin-polarisation effects
being conveniently accounted for by a previously established approximate
relation between the hyperfine coupling tensor and $W_\mathrm{a}$. For high
accuracy predictions of parity violating effects in radium fluoride the
necessity for systematically improvable correlation calculations is emphasised.
