Amos and Mary,
Not to deny the complexity of the human psyche or some wise individuals' ability to 'know' from their experience what a well-controlled study might demonstrate, but "common sense" is not as common as some think.
E.g., common sense still tells us that eye witness evidence is the soundest basis for judgment in criminal trials--to the point where our judicial system still gives it greater weight than "circumstantial" evidence and maybe even forensic evidence. Yet when eye witness judgments are tested in well-controlled studies, they turn out to be very unreliable (subject to distortion from many of the same factors you've mentioned as accounting for the complexity of us humans).
This illustrates why it's unwarranted to poo-poo scientific results when they "prove the obvious." What is "obvious" is not always right.