Why do some stocks have a higher margin requirement?
It is a question of how volatile the stock is perceived to be, its beta correlation to the S&P500 or other index. Margin requirements are derived from the Federal Reserve, Self Regulatory Organizations, the exchange itself, the broker you use, and which margining system you are using. So that makes this a loaded question. There are at least three margin systems, before you have your own risk officer in a glass room that doesn't care how leveraged up you get. Brokers primarily don't want to lose money. It's about how volatile the instrument is. Brokers are concerned not about you but about potential lawsuits stemming from their perceived inadequate risk management - letting you trade extremely volatile stocks with high leverage.  On top of that they run the risk of losing money in scenarios where a trader shorts a stock with all of the funds, the company rises 100% or more by the next day, in which case the trader owes money to the broker. If you look in detail you'll see that many of the companies with high margin requirements are extremely volatile pharmaceutical companies which depend heavily of FDA approvals.
The same applies if you were looking for a business to buy: would you pay more for a business that is doing well making increasing profits year after year, or for a business that is not doing so well and is losing money. A share in a company is basically a small part of a company which a shareholder can own. So would you rather own a part of a company that is increasing profits year after year or one that is continuously losing money? Someone would buy shares in a company in order to make a better return than they could make elsewhere. They can make a profit through two ways: first, a share of the company's profits through dividends, and second capital gains from the price of the shares going up. Why does the price of the shares go up over the long term when a company does well and increases profits? Because when a company increases profits they are making more and more money which increases the net worth of the company. More investors would prefer to buy shares in a company that makes increasing profits because this will increase the net worth of the company, and in turn will drive the share price higher over the long term. A company's increase in profits creates higher demand for the company's shares. Think about it, if interest rates are so low like they are now, where it is hard to get a return higher than inflation, why wouldn't investors then search for higher returns in good performing companies in the stock market? More investors' and traders' wanting some of the pie, creates higher demand for good performing stocks driving the share price higher. The demand for these companies is there primarily because the companies are increasing their profits and net worth, so over the long term the share price will increase in-line with the net worth. Over the short to medium term other factors can also affect the share price, sometime opposite to how the company is actually performing; however this is a whole different answer to a whole different question. Most brokers have a margin maintenance requirement of 30%.  In your example, it would depend on how much money you're borrowing from your broker on margin. Consider this: You have $250, and short AAPL at $500 on margin. This would be a common scenario (federal law requires investors to have at least 50% of their margin equity when opening a transaction). If your broker had a requirement of 30%, they would require that for your $500 position, you have at least $500 * .3 = $150 equity. Since you are currently above that number at $250, you will not be hit with a margin call. Say the price of AAPL doubles, and now your position is worth $1000. $1000 * .3 = $300, which is $50 above your initial equity. Your broker will now consider you eligible for a margin call. Most will not execute the call right away, you will often have some time to either sell/cover stock or add funds to your account. But not all brokers will warn you if you are breaking margin requirements, and sometimes margin calls can take you by surprise if you are not paying attention. Also, many will charge interest on extra margin borrowed. "A diversified portfolio (such as a 60% stocks / 40% bonds balanced fund) is much more predictable and reliable than an all-stocks portfolio, and the returns are perfectly adequate. The extra returns on 100% stocks vs. 60% are 1.2% per year (historically) according to https://personal.vanguard.com/us/insights/saving-investing/model-portfolio-allocations To get those average higher stock returns, you need to be thinking 20-30 years (even 10 years is too short-term). Over the 20-30 years, you must never panic and go to cash, or you will destroy the higher returns. You must never get discouraged and stop saving, or you will destroy the higher returns. You have to avoid the panic and discouragement despite the likelihood that some 10-year period in your 20-30 years the stock market will go nowhere. You also must never have an emergency or other reason to withdraw money early. If you look at ""dry periods"" in stocks, like 2000 to 2011, a 60/40 portfolio made significant money and stocks went nowhere. A diversified portfolio means that price volatility makes you money (due to rebalancing) while a 100% stocks portfolio means that price volatility is just a lot of stress with no benefit. It's somewhat possible, probably, to predict dry periods in stocks; if I remember the statistics, about 50% of the variability in the market price 10 years out can be explained by normalized market valuation (normalized = adjusted for business cycle and abnormal profit margins). Some funds such as http://hussmanfunds.com/ are completely based on this, though a lot of money managers consider it. With a balanced portfolio and rebalancing, though, you don't have to worry about it very much. In my view, the proper goal is not to beat the market, nor match the market, nor is it to earn the absolute highest possible returns. Instead, the goal is to have the highest chance of financing your non-financial goals (such as retirement, or buying a house). To maximize your chances of supporting your life goals with your financial decisions, predictability is more important than maximized returns. Your results are primarily determined by your savings rate - which realistic investment returns will never compensate for if it's too low. You can certainly make a 40-year projection in which 1.2% difference in returns makes a big difference. But you have to remember that a projection in which value steadily and predictably compounds is not the same as real life, where you could have emergency or emotional factors, where the market will move erratically and might have a big plunge at just the wrong time (end of the 40 years), and so on. If your plan ""relies"" on the extra 1.2% returns then it's not a reasonable plan anyhow, in my opinion, since you can't count on them. So why suffer the stress and extra risk created by an all-stocks portfolio?" Here is a simple example of how daily leverage fails, when applied over periods longer than a day. It is specifically adjusted to be more extreme than the actual market so you can see the effects more readily. You buy a daily leveraged fund and the index is at 1000. Suddenly the market goes crazy, and goes up to 2000 - a 100% gain! Because you have a 2x ETF, you will find your return to be somewhere near 200% (if the ETF did its job). Then tomorrow it goes back to normal and falls back down to 1000. This is a fall of 50%. If your ETF did its job, you should find your loss is somewhere near twice that: 100%. You have wiped out all your money. Forever. You lose. :) The stock market does not, in practice, make jumps that huge in a single day. But it does go up and down, not just up, and if you're doing a daily leveraged ETF, your money will be gradually eroded. It doesn't matter whether it's 2x leveraged or 8x leveraged or inverse (-1x) or anything else. Do the math, get some historical data, run some simulations. You're right that it is possible to beat the market using a 2x ETF, in the short run. But the longer you hold the stock, the more ups and downs you experience along the way, and the more opportunity your money has to decay. If you really want to double your exposure to the market over the intermediate term, borrow the money yourself. This is why they invented the margin account: Your broker will essentially give you a loan using your existing portfolio as collateral. You can then invest the borrowed money, increasing your exposure even more. Alternatively, if you have existing assets like, say, a house, you can take out a mortgage on it and invest the proceeds. (This isn't necessarily a good idea, but it's not really worse than a margin account; investing with borrowed money is investing with borrowed money, and you might get a better interest rate. Actually, a lot of rich people who could pay off their mortgages don't, and invest the money instead, and keep the tax deduction for mortgage interest. But I digress.) Remember that assets shrink; liabilities (loans) never shrink. If you really want to double your return over the long term, invest twice as much money. "The question mentions a trailing stop. A trailing stop is a type of stop loss order. It allows you to protect your profit on the stock, while ""keeping you in the stock"". A trailing stop is specified as a percentage of market price e.g. you might want to set a trailing stop at 5%, or 10% below the market price. A trailing stop goes up along with the market price, but if the market price drops it doesn't move down too. The idea is that it is there to ""catch"" your profit, if the market suddenly moves quickly against you. There is a nice explanation of how that works in the section titled Trailing Stops here. (The URL for the page, ""Tailing Stops"" is misleading, and a typo, I suspect.)" I would say the most challenging fact for this assertion is that HFT firms operate with extremely limited capital bases. For a stock with say 10m shares ADV, even a very large and successful HFT strategy might use a position limit of no more than 5000 shares.  That is to say if you sum up and net the buys and sells for a stock across the day the HFT firm will never exceed 10,000 shares (2x position limits assuming it completely flipped) on a stock that trades 10,000,000 shares on a given day. The high volumes are attained through high turnover, the strategy might trade up to 500,000 shares (or 5% of the volume) attaining a 50x turnover.  But that brings me back to the original point. In the market microstructure literature market impact generally has been found to scale linearly or even sub-linearly for net volume executed.   If I alternate between thousands of 1 lot buy and sell orders, it would be very difficult for me to move the market because the market impact of every one of my buy orders roughly cancels the market impact of my almost exactly equal number of sell orders.   There might be a higher-order mechanism at work, but I'm genuinely curious what you think it might be. How could strategies that attain such small net positions have such out-sized impact on market direction? As said by others, buying shares of a company will not support it directly. But let's think about two example companies: Company A, which has 90 % stocks owned by supporters, and Company B, which has only 1 % of stocks owned by supporters. Both companies release bad news, for example profits have decreased. In Company B, most investors might want to sell their stock quickly and the price will plummet. In Company A, the supporters continue believing in the company and will not want to sell it. The price will drop less (usually, but it can drop even more if the sellers of Company A are very desperate to get rid of the stock). So, why is it important for the company to have a high stock price? In the short-term, it's not important. One example is that the company can release more stocks and receive more financing by doing that. Other reasons are listed here: http://www.investopedia.com/articles/basics/03/020703.asp My interpretation of that sentence is that you can't do the buying/selling of shares outright (sans margin) because of the massive quantity of shares he's talking about.  So you have to use margin to buy the stocks. However, because in order to make significant money with this sort of strategy you probably need to be working dozens of stocks at the same time, you need to be familiar with portfolio margin.  Since your broker does not calculate margin calls based on individual stocks, but rather on the value of your whole portfolio, you should have experience handling margin not just on individual stock movements but also on overall portfolio movements. For example, if 10% (by value) of the stocks you're targeting tend to have a correlation of -0.8 with the price of oil you should probably target another 10% (by value) in stocks that tend to have a correlation of +0.8 with the price of oil.  And so on and so forth.  That way your portfolio can weather big (or even small) changes in market conditions that would cause a margin call on a novice investor's portfolio.