It would maybe be a step in the right direction, but motivation is going to be different between twins, often vastly different.

And I know how difficult it is to quantify "motivation", but really, I don't know a way around it.

But getting back to the issue: Is a college education worth the time, money and opportunity cost that we put into it?

And is it possible to design a study that could confirm or deny it?

The problem is that almost all motivated and/or smart 18 year olds go to college.

Is there a sample size large enough to find the ones that don't?

And even if you did, you would run into the opposite effect, motivated and smart 18 year olds that don't go college usually are outliers that have something else to do, like getting into a special program, a good job or being a professional chess player.

So, when you find these people, they would be the opposite sort of bias. Their circumstances would be so unique that they would prejudice the study against college.

Right now, college is the default option for people who can get in, so by its very nature, it biases the population.

I don't see a way around it, but I am open to ideas.