In your opinion, are introverts, extroverts, or those who linger in the middle more likely to succeed?

Both can be equally successful. Maybe not at the same thing, however. An introvert may be able to write an amazing novel or perfect hyper-realism in painting. These things require a great deal of time alone, in your own head. An extrovert may find success as a rock star or a headhunter. Being comfortable with crowds and confident approaching people would be something they would excel in.
Because there are so many ways to define success, regardless of personality characteristics, we all have a shot at finding it.

@GloPro : What a brilliant answer to my question!
That’s very helpful.

I think people who are ambitious, hard working, fearless, and feel a sense of wanting to prove something, and who push through setbacks are most likely to succeed. Introvert and extrovert never played much into how I thought of this topic. I think if someone is mismatched to what is necessary for their interests that can be a problem. For instance, if the are introverts, but their job demands they meet a lot of people and coordinate their work with a large team, that would be a problem.

My view is that extremism on either side is generally unhealthy and less likely to be successful in the long run.

Someone who is in the middle – not an flaming extrovert but not a mouselike introvert either – is more likely to bridge most situations.

In American society it is the extrovert who succeeds. The louder and more obnoxious, the more successful. Throw in a diagnosis of Sociopathy and you will surely go far. You might even become a CEO. I’m not kidding. There is viable research that backs this up.

On the other hand, in Scandinavia, it is the opposite. Calmness, rationality, grace under pressure, quiet competence combined with the sheepskin to prove it, are the valued characteristics. The loudmouth, the attention whore, the braggart, the strutting, ego-maniacal, over-caffeinated, macho boss who likes the smell of Napalm in the morning—no matter how well papered—is discounted immediately as a pile of bullshit and considered not effective because they are difficult to work with and no self-respecting person would follow their direction. But these types wall to wall in the US and rewarded for this obnoxious behaviour.

Extroverts certainly have the edge in certain professions like sales, public relations, customer service, etc. I am a well balanced extrovert. I am bright, witty, have a quick mind and am charming and able to easily engage others, have zero social anxiety.
Introverts often have more stick-to-itive-ness, are more focused, less bouncy brain stuff going and less likely to put their foot in their mouths because they think more before they speak, and are, often, better at follow through a lot of the time.

Their are +‘s and -‘s to everything. The older I get the more introverted I become, I think extroverts wear themselves out faster. haha

GloPro put it well. Being an introvert or extrovert will not define how you succeed. There are many more factors involved when defining success.

I found a long list of successful American introverts (there may be a non-American or two in this list), and this only includes actors and actresses, not businessmen like Bill Gates and Warren Buffett (introverts):

Famous Introverts — Actresses
Candice Bergen, actress, writer, photographer
Ellen Burstyn, actress
Glenn Close, actress
Audrey Hepburn, actress
Helen Hunt, actress
Diane Keaton, actress
Grace Kelly, actress
Jessica Lange, actress
Laura Linney, actress
Gwyneth Paltrow, actress
Michelle Pfeiffer, actress
Julia Roberts, actress
Meg Ryan, actress
Meryl Streep, actress

Famous Introverts — Actors
Clint Eastwood, actor/director
Harrison Ford, actor
Tom Hanks, actor
Jack Lemmon, late actor
Bill Macy, actor
Steve Martin, all around talented guy
Noah Wiley, actor
Other Famous Introverts
Johnny Carson, former Tonight Show host
Jane Clayson, host of CBS Morning Show
Matt Lauer, co-host on the Today Show
David Letterman, host of the David Letterman Show
Diane Sawyer, co-host of ABC’s Good Morning America
Barbara Walters, host of 20/20

Success is not dependent on introvert or extrovert.

It depends on what you are basing your definition of “success”.

@GloPro

Actually quite a lot of rock stars are introverts. They handle performing just fine because there’s a certain kind of disconnect between them and their audience. Small intimate settings they handle less well however.

I went into the Coloma comedy hour at my local pet/feed store today. I had several employees cracking up, one of the women said ” you are great!” haha
Now I am back to introvert, leave me the hell alone.
I have been watching old Johnny Carson clips and apparently, for all his humor and easy going persona he was incredibly socially bereft in actual gatherings of real people.

Interesting.