5.
Inverted-U curves have three parts, and
each part follows a different logic.^3
There’s the left side, where doing more
or having more makes things better.
There’s the flat middle, where doing
more doesn’t make much of a difference.
And there’s the right side, where doing
more or having more makes things
worse.^4
If you think about the class-size puzzle
this way, then what seems baffling starts
to make a little more sense. The number
of students in a class is like the amount
of money a parent has. It all depends on
where you are on the curve. Israel, for