One way to fix that problem is make the experiment last the entire lifetime of the "test subjects". It would be more expensive, but much more informative. They aren't worrying about what they'll do after the money runs out. Randomly pick X people who begin to receive their basic income at 18 years old and let them do whatever they want with it. You'd probably want to provide full health care too. Of course it's not a perfect experiment because they're surrounded by people who aren't receiving a basic income, which could affect their attitudes and actions in many ways.
And if you wanted to extend the experiment a bit you could create stipulations like, any literature, art, music, etc. that they produce must go into the public domain. (They could still make money off it in other ways if they want).
