Why is hybrid vigor often greatest for traits with low heritability?

In the context of animal breeding, hybrid vigor—or heterosis—refers to the increased performance of hybrid offspring compared to their parents, particularly in traits such as fertility and growth rate.1

The observation that hybrid vigor is often greatest for traits with low heritability is linked to the nature of those traits. Traits with low heritability, such as fertility, are heavily influenced by environmental factors and non-additive genetic effects rather than just additive gene action. Because these traits are often closely tied to an organism's overall fitness and survival, they are frequently subject to inbreeding depression in pure-bred lines.234

When two different, inbred lines are crossed, the resulting hybrids benefit from increased heterozygosity, which masks the deleterious recessive alleles that accumulate in inbred populations. Since low-heritability traits are highly sensitive to these genetic and environmental stressors, the "resetting" of genetic balance through hybridization provides a more dramatic improvement in performance than it does for highly heritable traits, which are already largely determined by additive genes and are less affected by the specific genetic combinations of the parents.423