This doesn’t affect the researchers’ argument. It’s stated only that “[s]ignificantly more” marmosets are left-handed than right-handed, which is compatible with the existence of many right-handed marmosets, provided there are also lots of left-handed marmosets out there.

This weakens the researchers’ argument. It implies left-handed and right-handed marmosets are frequently raised by the same parents, making it less likely that marmosets tend to acquire the same handedness as their parents.

This doesn’t affect the researchers’ argument. It doesn’t change the relative prevalence of left-handed marmosets, nor does it imply the researchers are basing their hypothesis on faulty evidence.

This implies humans tend to acquire their handedness from their parents, not that humans—or marmosets—acquire their handedness through imitation. There are many reasons, besides their tendency to imitate, why marmosets might, unlike humans, tend to be left-handed.

This supports the researchers’ hypothesis that handedness is developed through imitation, rather than solely genetic or environmental factors. It implies baby marmosets tend to acquire their handedness from other, nearby marmosets—not just from genetic relatives.