When I was in the seventh grade, a curmudgeonly teacher insisted all his students learn copperplate.

That made my handwriting worse - to the point that a couple of years later, at the same school, other teachers complained and I was sent to a remedial teacher to undo the damage inflicted by the earlier teacher who had wanted 'stylish'.

Now that you mention it, girls tend to have better handwriting.

I remember that at my school copies of notes taken by girls were particularly popular for that reason.

For me it seemed so obvious that I never wondered why it is the case. Or if it really is the case for that matter.

Girls definitely have better handwriting IME, and men and women differ stylistically too. When marking exams I can almost always correctly identify students as being male or female (or having learned to write in Chinese or Arabic, which each leave distinctive characteristics) before I see the name on the cover when I'm done.

What are some of the identifying characteristics?

Chinese has particular stroke orders that often carry over into English (e.g. cross the t before drawing the vertical) which leads to all kinds of differences when writing quickly, as the lines from the pen not leaving the paper are in different places. There are often also quite distinctive curving diagonal lines.

Arabic, being written right to left, requires quite a different hand position, which leads to very vertical strokes (not slanted to the right).

Some other distinctive features I've noticed are that beautiful cursive is likely to be someone Indian, and very small, fine, and vertical, likely Indonesian. I'm sure handwriting experts can tell a lot more than me.

What gave it away? The little hearts over the i's in "invariant"?

Sexism is out of style.