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Left-handed statistics

This blog entry by Tyler Cowen reminded me of the course that Seth Roberts and I once taught on left-handedness. The main things I remember learning:

1. Left-handedness is not the opposite of right-handedness. Most righties do everything with their right hand, but lefties are mostly mixed. Also, left-handers are typically OK with their right hands, but righties are typically not so good the other way. Related to this is that there’s really not such a thing as “ambidextrous”: the term “mixed-handed” is better: people who use different hands for different tasks are usually OK with either hand.

2. This is more of a “folk psychology” thing, but it’s interesting: a lot of people, especially lefties, either want to know the “rule” for determining whether someone is left-handed, or think there is such a rule. Many people aren’t comfortable with the idea of a continuum, and want this to be a binary variable. (Interestingly, I even ran across a statistics textbook once that (mistakenly) characterized handedness as an example of a categorical variable.)

3. The studies that find lefties to die younger are interesting. Not airtight, but not trivially demolishable, either. At least as of my reading in 1994, the case is still open on this one.

4. We did a little study in our class (approx 20 students, about 1/4 righties and the rest left- or mixed-handed), asking each student to make a list of his or her closest friends (outside of the class itself) and then give them the handedness inventory (a standard 10-question battery that yields a handedness score between -1 and 1). We found a statistically significant correlation between the handedness of the people in the class and the average handedness scores of their friends. We never followed this up with further studies, though.

5. In reading the papers for the class, I noticed that many were written by scientists from Canada and New Zealand, not much from the U.S. I asked Seth why, and he said it’s because you can study handedness with a low budget.

6. We were featured in the local papers as an example of a fun college class. But there was one media outlet that contacted us, I don’t remember which one, which Seth suspected was trying to use us as an example of the crap that gets taught in college nowadays. I was careful to be very boring when talking with this reporter so that he wouldn’t get any incriminating quotes from me. Also, a local TV station wanted to come and shoot one of our classes, but they decided not to when I explained that we weren’t really focusing on original research–the course was mostly discussions of existing articles. (It was a good class, though, I think.)


  1. Frank says:

    Could you post a list of the papers you assigned for your handedness course? I'd be very interested to have a look a them.

  2. deb says:

    1. When and why did you teach classes with Seth Roberts?

    2. I reviewed and funded several interesting grant proposals on handedness and decision biases (like framing) when I worked as the DRMS PO @ NSF. I can't remember the PIs names off the top of my head, but there are two psychologists in the midwest who claim that handedness is a predictor of susceptibility to kahneman and tversky type biases.

    I am left-handed. I have noticed over the years that the proportion of left-handed decision scientists (Paul Slovic, others) is much higher than you'd statistically expect.

    I was disappointed in the researchers' method of defining handedness. They opted to go for a kind of continuum of how many things you do with each hand. But there's asymmetric social pressure – there's a LOT of pressure to do stuff right-handed. That dog expert guy in canada has shown that lefties die about 7 years earlier than righties.

    So "mixed" lefties are probably mixed because of social pressure but mixed righties are probably true mixies.

    Interesting topic, that's 4 sure.

  3. Andrew says:

    We taught the course about 12 years ago. It was part of an program of freshman/sophomore seminars at Berkeley. I'd read the book on left-handedness by Coren (the "dog expert guy") and thought it would be a fun topic for a course. But I know next to nothing of psychology, so I called Seth and asked if he knew anyone who might want to co-teach the class. It turned out he wanted to do so himself.

    The students went through one chapter a week of the Coren book and also read relevant articles from the scientific literature. I'll try to dig up the list–they were mostly things referred to by Coren, I think.

  4. Michael Anes says:

    3. The studies that find lefties to die younger are interesting. Not airtight, but not trivially demolishable, either. At least as of my reading in 1994, the case is still open on this one.

    This absolutely sounds like it was an interesting course, and Seth Roberts' name sounds deeply familiar to me…Michigan State or Boston University…? I'll have to puzzle that one out…

    But on point, I tend to believe Lauren Harris's arguments in his rebuttals to Coren and Halpern (those about considering year-of-birth cohort, the null results found with even slightly larger samples of baseball players, etc…).

    I'm liking your blog quite a bit by the way!

  5. Jim P says:

    This is a little late, but here are two other recent comments on handedness in sports:
    why there are lots of lefties in hockey

    and bats left, throws right

    One of the theories is that it is based on when you pick up a sport. "The top hand on a hockey stick has to be able to handle the torques of a hockey stick while the bottom hand just has to handle the weight with no torques. Since the torque (the shaft being a lever) can be rather hard to handle, at a young age one has to use their strongest hand to handle the torques thus a youngster (say 4 yr old) will use his strong hand (generally right) to hold the top of the shaft, thus they will learn to shot left if right handed. "

    This might be why my son, who started swinging a bat and golf club at age two, naturally swings lefty but does everything else righty.

    Anyway, thought this might be of interest.

  6. Patricia says:

    Since I am left-handed, I am intrigued by the thought that lefties die earlier than right-handed people. Left-handed individuals are not an oddity to be studied. Children who are left-handed should be taught the same as a child who is right-handed.