26 May 2011

A quick in-class critical thinking demo

In one of my recent classes, I wanted to start off with a quick exercise to get people thinking about hypothesis testing and critical thinking. I used a demo that are used to hawk balance and power bracelets and “applied kinesiology.”

In brief, you have someone stand on one foot with both arms extended sideways. You press down on one arm, and they tend to fall over. Then, you give them a gizmo of some sort (I used a rubber band), and repeat the test, and people find they don’t fall over anywhere near as easily. A video is below; another quite good one is here.

The trick is a very slight difference in the direction you press. To make them fall off, you press slightly outward from their body; to keep them balanced, you press every so slightly towards their body.

I did this in class, having seen the video but never having done this before. I went through four or five different volunteers, and was able to get the effect every time.

Nobody believed that my rubber band was the cause of the difference. When I asked people for ideas as to what was going on, several of them could be easily tested on the spot. One person suggested I had placed my hands in different locations in the two tests. I got a new volunteer, placed my hands very precisely in the same spot, and was able to tip or not tip them at will.

One suggested a placebo effect. Problem: None of the people I gave the rubber band to believed the rubber band was the cause, which made it hard to argue that it was a placebo.

We also talked a bit about the mechanisms that some bracelet makers propose for how their bracelets are supposed to improve balance. Some invoked things like “natural energy fields” or “life energy,” which was a great lead in to talk about how living things differ from non-living things. Do living things have some sort of energy that non-living things don’t?

Eventually, one person near the very front of the class figured out what I was doing. I think that with a little practice, though, it would be very difficult to detect.

In summary, this is easy to do, requires nothing but a student volunteer, and is a great jumping off point for all kinds of discussions about hypothesis testing, evidence, mechanisms, and lots more.

That you get to push people around is just a bonus.


Spiral Stepper said...


I actually just ran into one of these demos at the mall, and I have to admit that it threw me for a loop. I was positive that there was no possible way a little silicone band with a chunk of metal in it could affect my balance so significantly -

found your blog by googling "mind tricks balance stand on one foot press down on the arm" xD

Thanks for explaining it so well, I'll be sure to point out what you've said to anyone else tempted to buy one of those bracelets ;D

TheCellularScale said...

What a great video! It certainly shows the importance of a control group when making claims about anything.

Zen Faulkes said...

I just did this demo in my class again today, and it worked even better. Nobody twigged to what I was doing. Interestingly, the students, like last year, almost all immediately went to the idea that it was some sort of placebo effect.

When I asked for them to suggest tests, they made several good ideas that involved changes in the placement of the band, placement of the subject... but none suggested changing the tester (me!).

Cellular, more important than a control group in this situation? The concept of a double blind test. If I had to do this without knowing if the person had the band, the jog would be up.

TheCellularScale said...

Yes a double-blind test would be perfect.

I think it would be interesting in your class to have someone else try it. The man in the video seems to give the shop demonstrators a lot of credit, saying that they are unconciously pressing differently on the arm. I wonder if that is actually true. The alternative being that they are knowing scam artists. It would be interesting if a volunteer from your class (after seeing the demonstration) might unconsciously press more inward on the arm because s/he is expecting resistance.

This trying to bias the experimenter might work better if instead of a rubber band it is something more high-tech looking with some pseudo-scientific reason for balance. (Something ferrous that 'interacts with the magnetic field of the earth', or headphones that play 'brain wave altering pulses'. Haha, you could have lots of fun thinking up BS like this.) It would even be interesting to see what specific classes of BS are most believable and if anything is strong enough to induce some student to actually press differently on the arm.