Stuck on Q3 and Q4.
Can't find any special names for that kind of diagram in Q3.
Q4 has a typo I think, but even so, can't find a theory that causes it to happen, even though I know what the effect is.

Q3 help:
The _ curve is an economic theory, proposed in 1996 by American economist ___, which indicates that there is a level of government spending that maximises economic growth. The theory is used by classical liberals to argue for a decrease in overall government spending and taxation. The U-shaped curve suggests that the optimal level of government spending is 15–25% of GDP.

Found this text on the Matthew effect, which makes it unclear what's behind it.
"The emergent collective modes of behaviour are due to the heterogeneity of the interaction patterns, the presence of nonlinearity and feedback effects, and it is here were the reasons behind the Matthew effect ought to be sought. This, however, raises the question whether the Matthew effect is due to chance or optimization."

So I'm not sure if there's another source you have that clearly indicates a specific theory, but from that read it sounds like there is no clear one.

Also having huge trouble on matthew effect (Q4). I know what it is, I just can't find what you want as an answer...
Wikipedia is useless on this.

EDIT: SOLVED after all. I think the matthew effect only worsens this question and makes it harder to find... Only when I ignored it did I find the answer lol.

thank you! :)

Hey!! I remember how much fun it was having you in the Science event as a puzzle creator... here's a chance at another event I'm doing-- hope you can return with designing a puzzle!!