I think next step should be developing a test that can predict how someone will react to it.

Unnecessary: foolish people always gonna fool. Anyone that far gone in the lacking judgement department demands far more help than anyone can reasonably be expected to provide, and attempting to “foolproof” for them will only drag everyone else down while doing nothing for them. Likewise, just because some people overeat junk food doesn’t mean we need to devise some test to decide who can safely get junk food: it’s a personal choice, the risks of bad judgement are reasonably understood, & that bullshit’s beyond paternalistic.