James' comment, "This depresses me, I would like to believe that there is something in our art that makes people better, not that it just attracts people of a certain type." and all the responses which focused on comparisons to sports got me thinking.
First, rather than compare martial arts practice to football, soccer, or another team sport, I think we should try to compare to other individual sports. Two that I have personal experience with are fencing and table tennis. I also have some experience with gymnastics through my children. By virtue of being individual events, a lot of the accountability question is resolved. If you lose, there's almost no one else to blame. Of course, in fencing or gymnastics you can blame the director or the judges. In table tennis you can blame it on your paddle. But when you reach any sort of proficiency, you realize only you can take responsibility for the outcome.
I also think that gymnastics, fencing, and table tennis (in decreasing order) all teach some body awareness that helps with balance and body coordination that Shannon writes above.
But, I do think there is something fundamentally different between martial arts and these individual sports. Unique in the martial arts experience is the constant interaction with fear. And not just fear of inanimate objects like bars and gravity as in the case of gymnastics, but fear in the interaction between human beings. In kumite, we need to face our own fear of getting injured. We need to fear for the safety of our partners - for the consequences of our own attacks. I think that it is in facing the many fears on a repeated basis that forces many of us to look at ourselves and see where our character is weak or tarnished. (It seems that this fear should exist in fencing, but I don't remember feeling afraid. There are rules about excessive force. In many ways, it's only a game. Oversimplified, if you can get your weapon to make your light go on, you get a point.)
Now here is where things diverge. I think the difference between good sensei and bad will be in what they decide to do when the student sees something in themselves they're not happy with. The reaction I would expect in the 'street fighting fu' schools would be to tell the student to ignore it and "be tough". If the student stays in this type of school he will just adds another layer of protection around the unhappiness and bury it away. The problem won't be solved and the character won't change. Another day, in another way, the same thing will come back up and out. On the other hand, a good sensei will understand what the person is going through and offer support and encouragement as the student explores what's bothering them. There may even be other exercises within the art that can help the person see things from a different perspective. This causes change in the character.
So, it's not black and white: sport vs martial arts. Even if the martial arts are special the sensei and/or the tradition of the art will play a large part in whether or not change occurs.
Lastly, I don't think it's black and white between self-selection and change caused by the martial arts.. People who decide to study the martial arts have to think that there's something "fun" about putting themselves into a fearful situation (in the form of kumite). Something about that has some initial appeal to them whether it's to build confidence, to defend themselves, or to fight others. Whether or not their initial reason matches the sensei's style at the school they attend will determine whether or not they continue long enough to find themselves with fear, uncertainty, and doubt. And once they do, their sensei's reaction will be another determining factor about whether or not they continue.
Seems like an awful lot to right to in the end say there's still no easy answer to the question.
I just started reading a weblog called Karate Talk and came across a new posting about Karate and Character.
Interestingly enough, there was a similar article I just read in the latest issue (Volume 13, Number 3) of the Journal of Asian Martial Arts. The article, "Gravitation Versus Change: Explaining the Relationship Between Personality Traits & Martial Arts Training" is written by Brandon Seig, M.S. Here is the abstract for the thirteen and a half page article (with two and a half page bibliography):
Martial arts practitioners frequently cite their training as being responsible for key areas of self-improvement including the mental and emotional realms. Skeptics of the martial arts also believe that training can induce a change in the students but disagree with the outcome being positive. Both arguments are typically based more on personal testimony or opinion than academic theory or research.Can martial arts training actually induce a change in personality within the practitioner? In sport psychology, this notion, referred to as the change hypothesis, is in contrast to the gravitation hypothesis, which states certain personalities are attracted to the martial arts and are more conducive to succeeding at the martial arts. A literature review of existing research was merited to determine which hypothesis is valid. Many of the studies that were reviewed paint a varied or even contrasting view of the personality traits of martial artists. Many studies from both camps suffer from being merely descriptive and not explanatory, a consequence of not being longitudinal over time. Supporters of either hypothesis in these types of studies appear to presume one or the other to be the case. Yet, there are a benchmark studies that seem to offer more concrete insights, and the martial arts community should take note of them. These studies not only suggest that the martial arts can indeed have positive effects on personality, but also imply the outcome is contingent upon proper teaching which includes "traditional" martial arts values and philosophy.
As you can probably guess from the abstract, it's a fairly academically written article. I think it did a great job of being a "literature review" as it set out to be. You are in for a disappointment if you expect a clear answer to the question to be found. The author says in his closing thoughts that, "Careful and more rigorous examination is required before the martial arts' proposed ability to bring about positive personality changes can be validated."
In my opinion, I think the question is too broad to be answered very scientifically. There's far too many different kinds of martial arts and far too many different teachers and teaching styles for this to be decided scientifically.
Personally, I've found that practicing causes personal change. It usually causes me to reevaluate "fixed ideas" I've had. Makes me see where they've come from and over time, helps me to find ways to change them.