Today, we continue our discussion of a few of the most commonly repeated, yet subtly dangerous, pieces of Wall Street advice.

We've already seen how doubling down can be Foolish (with the big "F") and how sticking to buying what you know can stick it to your portfolio. Let's now look at three more of the dangerous half-truths.

One "magic" formula for success
Perusing the magazine rack at Barnes & Noble or listening to guests on BubbleVision, you'd think that somewhere out there is the magic formula for instant and everlasting success. Or at the very least, it seems that a large proportion of the money managers willing to talk at length about their strategy are convinced that they and they alone have the "one right approach."

But consider this -- both Warren Buffett and George Soros have built considerable wealth with very different philosophies. Peter Lynch built a fantastic investment record and owned prodigious numbers of stocks along the way. Compare that with Bill Miller (or Warren Buffett, for that matter) and the relatively modest number of stocks held in his mutual fund's portfolio at any point in time.

It amazes me how much time that otherwise smart folks will waste arguing about whose methodology is best. If you're both making money and both earning good returns on your money relative to the risk you're taking, what's to argue about? Different approaches work in different market environments, different approaches offer different trade-offs of risk and reward, and different approaches make different demands on your psyche.

For instance, some investors will prosper by building portfolios of risky stocks and hoping for the occasional Neurocrine Biosciences (NASDAQ:NBIX), NetEase (NASDAQ:NTES), or SanDisk (NASDAQ:SNDK) to outweigh the inevitable mistakes. Others will focus relentlessly on valuation and/or cash flow and fill their portfolios with the likes of Procter & Gamble (NYSE:PG) and Wells Fargo (NYSE:WFC). One investor might see more volatility and perhaps higher returns over time, but the other might sleep easier and/or be happier with lower-risk trade-off and more consistent results.

Forget trying to figure out the "one right approach," and instead figure out the methodologies that work for you. Some people can't resist the lure of growth, and some, like the Fool's own Rick Munarriz and Charly Travers, seem to have an eye for panning out those nuggets of gold where many people see piles of gravel. Others follow tried-and-true value principles that depend heavily on cash flows and discounted valuations. And then there are those, like yours truly, who mix a bit of both into a personal style that may be confusing to others (but works just fine for the user, thank you very much).

Anybody can do this
I honestly do believe that any adult capable of graduating from high school can learn enough about financial markets to take some measure of control over his or her investments. As is so often the case, though, the issue doesn't come down to pure intellectual horsepower; it involves attitude and psyche as well. If you're undisciplined, overemotional, or impatient, all the smarts in the world will come to naught.

Don't believe me? Well, consider this -- it's something of a shared joke in the investment world that doctors and lawyers often make some of the worst investors of all. It isn't because they're not smart; it's because they lack some number of the other attributes (like humility) that you need for success. If doctors would stick to the Medtronics (NYSE:MDT), Mercks (NYSE:MRK), and such of the world, they'd probably do better; it's when they confuse an expertise in podiatry with special insight in semiconductors (and so don't bother to do real due diligence) that they often go astray.

The trouble is that some people are just not especially psychologically equipped for long-term market success. People get fearful when they should get greedy (often when an industry has been abandoned by managers and analysts -- and appears to be flat on its back), and they get greedy when they should be getting fearful -- as anybody who rode Cisco (NASDAQ:CSCO), Qualcomm (NASDAQ:QCOM), et al. down from the tech-bubble highs can appreciate.

Humans are also naturally social and have something of a herd instinct -- most investors want to own stocks that will earn them approving nods during happy-hour bull sessions, not stocks that elicit snorts or blank stares. So that works to steer people away from the small and undiscovered -- areas where a lot of people (including our Motley FoolHidden Gems team) have made oodles of money over the years.

What doesn't kill us ...
Last and not least, human beings in general (and investors in particular) seem to remember pain more easily than pleasure. In fact, I once had a gym teacher who espoused this very philosophy to our decidedly multilingual gym class -- "you might not understand English, but I'll bet you understand pain." More than a few investors get burned by a poor decision or two and then decide that the whole thing is some rigged game that can't be won.

But like a kid who quits hockey after taking his first hard check, you never get anywhere when you quit after a major setback. When most of us here at The Motley Fool lose money on a stock, our first move is to examine what we did wrong. Other people, though, look to blame the current stock market bogeyman du jour -- foreign investors, program trading, naked shorts, malicious elves, or Lord Voldemort.

It also takes time, energy, and effort to be really successful in investing. I'd love it if I could spend just an hour each day and trounce the market, but that's not the reality for me, and I suspect it's not the reality for most people. Unlike me, some people actually want to have real lives and would rather remember the birthdays and phone numbers of loved ones, as opposed to the trailing cash flow numbers for Tyco (NYSE:TYC). Consequently, some people just aren't able (or willing) to give investing the time and attention needed for success to fully bloom.

Until next time
We're now two-thirds of the way through my brief tour of the six market half-truths that I think are the most potentially damaging to you, the individual investor. We've seen the danger in ignoring good shares at momentarily cheap prices, and we've seen how bickering about the one true path in investment management is a waste of time. Still to come, though, are two more half-truths that can harm your financial health.

For more myths debunked:

NetEase is a Motley Fool Rule Breakers recommendation. Merck is a Motley Fool Income Investor recommendation. Tyco is a Motley Fool Inside Value recommendation.

Fool contributor Stephen Simpson has no financial interest in any stocks mentioned (that means he's neither long nor short the shares).