Proponents of efficient market theory believe that all known information is priced into a stock or other investment product. The advent of algorithmic trading makes that process almost instant, according to EMT theorists. This was seen on April 23, 2013, when a fake tweet that claimed the White House was bombed and President Obama injured immediately sent markets crashing. Although the market recovered within four minutes, this incident exposed the network of supercomputers that constantly scan headlines looking for keywords that are programmed to sell equity positions when the right words are flagged.

This would seem to prove that EMT is true - especially now that non-human traders dominate the market. However, others say, "not so." They argue that the reason long-term investors like Warren Buffett, as well as high frequency traders, can profit is because of market inefficiencies. These inefficiencies are natural because the markets are comprised of humans or computers programmed by humans.

Regardless of how disciplined, humans often trade with behavioral biases that cause them to act on emotion. This is the basis of behavioral finance, a relatively new field of study that combines psychological theory with conventional economics. Behavioral finance predicts trading behavior and is used as a basis for creating more efficient trading strategies. A University of California study found strong evidence that investors have behavior biases that often affect investing decisions more than empirical data. Here, we highlight four of those biases that are common among retail traders who trade within their individual brokerage accounts.

Overconfidence has two components: overconfidence in the quality of your information, and your ability to act on said information at the right time for maximum gain. Studies show that overconfident traders trade more frequently and fail to appropriately diversify their portfolio

One study analyzed trades from 10,000 clients at a certain discount brokerage firm. The study wanted to ascertain if frequent trading led to higher returns. After backing out tax loss trades and others to meet liquidity needs, the study found that the purchased stocks underperformed the sold stocks by 5% over one year and 8.6% over two years. In other words, the more active the retail investor, the less money they make. This study was repeated numerous times in multiple markets and the results were always the same. The authors concluded that traders are, "basically paying fees to lose money."

How to Avoid This Bias
Trade less and invest more. Understand that by entering into trading activities you're trading against computers, institutional investors and others around the world with better data and more experience than you. The odds are overwhelmingly in their favor. By increasing your time frame, mirroring indexes and taking advantage of dividends, you will likely build wealth over time. Resist the urge to believe that your information and intuition is better than others in the market.

Reducing Regret
Admit it, you've done this at least once. You were confident that a certain stock was value priced and had very little downside potential. You put the trade on but it slowly worked against you. Still feeling like you were right, you didn't sell when the loss was small. You let it go because no loss is a loss as long as you don't sell the position. It continued to go against you but you didn't sell until the stock lost a majority of its value.

Behavioral economists call it regret. As humans, we try to avoid the feeling of regret as much as possible and often we will go to great lengths, sometimes illogical lengths, to avoid having to own the feeling of regret. By not selling the position and locking in a loss, a trader does not have to deal with regret. Research shows that traders were 1.5 to 2 times more likely to sell a winning position too early and a losing position too late, all to avoid the regret of losing gains or losing the original cost basis.

How to Avoid This Bias 
Set trading rules that never change. For example, if a stock trade loses 7% of its value, exit the position. If the stock rises above a certain level, set a trailing stop that will lock in gains if the trade loses a certain amount of gains. Make these levels unbreakable rules and don't trade on emotion.

Limited Attention Span
There are thousands of stocks to choose from but the individual investor has neither the time nor the desire to research each. Humans are constrained by what economist and psychologist Herbert Simon called, "bounded rationality." This theory states that a human will make decisions based on the limited knowledge they can accumulate. Instead of making the most efficient decision, they'll make the most satisfactory decision. 

Because of these limitations, investors tend to consider only stocks that come to their attention through websites, financial media, friends and family, or other sources outside of their own research. For example, if a certain biotech stock gains FDA approval for a blockbuster drug, the move to the upside could be magnified because the reported news catches the eye of investors. Smaller news about the same stock may cause very little market reaction because it doesn't reach the media.

How to Avoid This Bias
Recognize that the media has an effect on your trading activities. Learning to research and evaluate stocks that are both well-known and "off the beaten path" might reveal lucrative trades that you would have never found if you waited for it to come to you. Don't let media noise impact your decisions. Instead, use the media as one data point among many.

Chasing Trends
This is arguably the strongest trading bias. Researchers on behavioral finance found that 39% of all new money committed to mutual funds went into the 10% of funds with the best performance the prior year. Although financial products often include the disclaimer that "past performance is not indicative of future results," retail traders still believe they can predict the future by studying the past.

Humans have an extraordinary talent for detecting patterns and when they find them, they believe in their validity. When they find a pattern, they act on it but often that pattern is already priced in. Even if a pattern is found, the market is far more random than most traders care to admit. The University of California study found that investors who weighted their decisions on past performance were often the poorest performing when compared to others.

How to Avoid This Bias
If you find a trend, it's likely that the market identified and exploited it long before you. You run the risk of buying at the highs - a trade put on just in time to watch the stock retreat in value. If you want to exploit an inefficiency, take the Warren Buffett approach; buy when others are fearful and sell when they're confident. Following the herd rarely produces large-scale gains.

The Bottom Line
Do you see a bit of yourself in any of these biases? If you do, understand that the best way to avoid the pitfalls of human emotion is to have trading rules. Those might include selling if a stock drops a certain percentage, not buying a stock after it rises a certain percentage and not selling a position until a certain amount of time has elapsed. You can't avoid all behavioral bias but you can minimize the effect on your trading activities. (For related reading, see "Cognitive vs. Emotional Investing Bias")