> The Prolific advert stated that participants were being sought for an anonymous survey on their views on the use of modafinil as a cognitive enhancer by UK university students.

Textbook case of "how do we select the most number of people who would say yes" ism. People who are taking modafinil and have a strong opinion about it.

If I'm a student I would never click on that ad in the first place if I have never heard of modafinil (like most people haven't), because the survey is about something I don't know. And if I vaguely heard of it and click, there's a friendly awareness page and quiz right at the start about the thing and its side effects that says I won't get paid if I don't answer correctly, what are the chances I'll continue wasting my time?

I'm sure all 12 of them are.