You just answered your own question. Someone that has had money problems is more susceptible to taking bribes, and once they've taken a bribe, they are now more

You just answered your own question. Someone that has had money problems is more susceptible to taking bribes, and once they've taken a bribe, they are now more susceptible to blackmail. As a question along the lines of "have you ever taken a bribe?" is unlikely to get answered honestly, debt problems are used as a proxy.