  We explore the physical origin and the general validity of a propensity rule
for the conservation of the hyperfine spin state in three-body recombination.
This rule was recently discovered for the special case of $^{87}$Rb with its
nearly equal singlet and triplet scattering lengths. Here, we test the
propensity rule for $^{85}$Rb for which the scattering properties are very
different from $^{87}$Rb. The Rb$_2$ molecular product distribution is mapped
out in a state-to-state fashion using REMPI detection schemes which fully cover
all possible molecular spin states. Interestingly, for the experimentally
investigated range of binding energies from zero to $\sim13\:\textrm{GHz}\times
h$ we observe that the spin-conservation propensity rule also holds for
$^{85}$Rb. From these observations and a theoretical analysis we derive an
understanding for the conservation of the hyperfine spin state. We identify
several criteria to judge whether the propensity rule will also hold for other
elements and collision channels.
