In

Taka, in is not needed in that sentence. It is not adding to or modifying anything in the sentence. If you ever find yourself questioning something like this, just ask yourself: "In what?"

Since in cannot answer for itself, it's not correct to include.

Isn't that usage of a preposition similar to this kind?

Where are you going to?

It is and technically it would be wrong if you were to use that written. It's common for people in the US to add an irrelevant 'to' to the end of that question when speaking.

So it's the same kind of usage which is common but not preferable. I see.

Actually, the sentence is from a written text and it seems to be a serious one about language. Hmm...

The written text probably meant 'projected inward'. That would make sense.

The sentence in question is followed by this. So I don't think it's inward.

It may indicate aspects of our gender, ethnic background, nationality, social class, occupation, education and personality.

Well in that case, they just made an error.

I see. Thanks, Maku!