That's hard to test because you might kill people or go to jail in the process, not because it's simply hard to test. Also, concluding through controlled study

That's hard to test because you might kill people or go to jail in the process, not because it's simply hard to test. Also, concluding through controlled study that alcohol consumption impairs your ability to safely control an automobile is much easier than concluding that caffeine intake impairs creativity. Creativity for one thing can be highly influenced by inspiration. Driving ability not so much.