English teachers do the same thing for detecting plagiarism. Bring the kid into your office and talk about the essay. If he's completely clueless, someone else wrote it for him.
Overall, I get more value from tests that are constructed so that I can learn positive things about the candidate - as many opportunities as possible for the candidate to distinguish themselves. If the only reason for a particular approach was to provide the opportunity for immoral candidates to weed themselves out by committing obvious plagiarism (negative data), then there's probably a better approach that tells me more about the candidate per unit time.
If I was going to continue, I would use a problem that is (1) more representative of the actual work being done by the team; less of a puzzle (2) custom designed for the team or company; not a preexisting or well known problem. (Even candidates who don't cheat can have an unfair advantage on well-known problems if they have coincidentally encountered it before! Another reason to use unique questions.)
This reminds me of the debate over whether performance-enhancing drugs should be allowed in mathematics. Why do you think it's so important for the candidate to personally invent every aspect of their solution? What if you just told people that it's ok to use external resources to solve the problem?
A class might give exams in any of these ways:
- exams only happen in class, where everyone can notionally be supervised
- exams are take-home, but you can't read the textbook while you're taking one
- exams are take-home, and you're free to read the textbook
There's cheating under all of those models, including the first one which takes the form that it does specifically to prevent cheating. The implicit goal (for the students) of model 1 is to make sure they've internalized whatever is being taught. The implicit goal of model 3 is to make sure that, even if they haven't internalized the material, they're capable of applying it. The implicit goal of model 2 is to make sure they'll comply with arbitrary, unenforceable demands (in this context, usually called "the Honor Code"). That might make sense if you're hiring a cashier -- but is it really your first priority?
Do you feel similarly that the implicit goal of this model is "to make sure they'll comply with arbitrary, unenforceable demands" and still not to test internalization of the material?