The whole reason monkeys even enter the picture is because the society currently unfairly favors a hypothesis that it's all due to socialization, and not at all due to biology. Any study that attempts to show biological aspects only results in the supposed socialization event resulting in the difference being pushed earlier in the development.

The studies showing that female infants are more interested in looking at pictures of faces, while male infants are more interested in looking at pictures of mechanical toys wasn't enough, so researchers are one upping and studying monkeys. It does not help though, I heard from someone saying this in good faith that the sex differences in toy preferences in monkeys are due to societal norms in monkey groups.

At this point I'm seriously starting to consider the socialization hypothesis (which is obviously true to some degree, i.e. socialization plays a huge role in the observed sex differences) to be unscientific, that is, unfalsifiable.