Marmosets are the only primates other than humans known to display a preference for using one hand rather than the other.
Conclusion
Marmosets learn which hand to use by imitating their parents (if your parent is left handed, then you'll imitate your way into being left handed)
Evidence
Infant marmosets engage in lots of imitative behavior.
Evaluate
Any time we see the word hypothesis in the question stem, we know it will be a Causal conversation, so we can almost always organize it in our usual Curious Fact / Explanation style:
Curious Fact: Why are marmosets the only non-human primates that prefer one hand over the other? How do they develop a preference for one hand?
Author's Story: They engage in a lot of imitative behavior, so if they're imitating a left-handed parent then they'll end up left handed.
The two questions we always ask: - How ELSE could we explain the curious fact? - How plausible is the author's story?
For Strengthen questions, we sometimes rule out a different way to explain the curious fact, but usually we add some plausibility to the author's story.
The most common Strengthener in causal arguments is a No Cause, No Effect strengthener. Since the author thinks that imitative behavior leads to hand preference, any answer structured like would strengthen.
It could be that other primates (who don't have hand preference) don't have imitative behavior as much. It could be that marmosets who grow up without a parent to imitate end up having no hand preference.
A "more Cause, more Effect" strengthener would give us other instances of imitative behavior being joined with hand preference (such as establishing that humans ALSO engage in much imitative behavior as kids).
Goal
Look for something that makes it seem more likely that marmosets learn hand preference through imitative behavior.