What does "How these expectations differ is in part a function of culture."

The preceding sentence is " As mentioned earlier, people have expectations for intellectual performances that differ for children of different ages. " mean?

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@celia_mae_wazowski87 The short answer is no, in this case, neither “that” nor “which” can be added between “is” and “in part”.

The longer explanation is that there are some sentences that could use the phrase "is that in part," but they are few and far between. Ultimately, even if these sentences are theoretically grammatically correct, they are excessively verbose and hard to use in a way that sounds natural. In general, you probably won't see very many, if any sentences like this, so this explanation is likely to not be very useful, but there might still be some value in seeing "in part" used in different ways.

That being said...

Another word that can replace "in part" is "somewhat", so the sentence could also be rewritten as: "How these expectations differ is somewhat a function of culture."

"How these expectations is that somewhat a function of culture." ❌
"How these expectations is which somewhat a function of culture." ❌

Although neither of these sentences are correct, we can use "is that somewhat" and "is which somewhat" as analogs for "is that in part" and "is which in part".

Since "is which somewhat" is not a phrase that's used, we can rule out being able to use the phrase "is which in part".

However, "is that somewhat" is a phrase that can be used, and is commonly followed by an adjective or adverb.

"A long charging time is that somewhat unavoidable issue that electric car manufacturers have yet to overcome" (adjective)

"Cognitive Science is that somewhat quickly growing field that spans the gap between Computer Science and Neuroscience" (adverb)

With that in mind, we can construct a similar sentence that uses the phrase "is that in part":
"Tradition is that in part nebulous facet of culture that connects people with their forebears."

This sentence could be improved somewhat with commas:
"Tradition is that, in part nebulous, facet of culture that connects people with their forebears."

And finally, to return to the original sentence:
"How these expectations differ is that in part unpredictable consequence of cultural variety."

Disclaimer/Side note: "somewhat" and "in part" are not perfectly interchangeable. There are plenty of examples where "in part" cannot replace "somewhat". I can't think of any examples at the moment where "somewhat" replacing "in part" would be wrong, but there are cases where it would sound strange.

This drink is somewhat cold. ✅
This drink is in part cold. ❌

The softness of this cake is in part due to the kind of flour that was used. ✅
The softness of this cake is somewhat due to the kind of flour that was used. ⚠️ (Not incorrect, but not the best choice of words)