How to decide when to sell shares

Investors spend a lot of time thinking about buying shares. But they should consider their selling strategy too.

When it comes to investing in shares, there's plenty of advice on how to identify companies to buy. But there's a relative dearth of advice on the other big transaction involved in successful investing knowing when it's time to sell. This is a pity, because selling too early can be as bad, if not worse, than selling too late. As the FT noted a while ago, even the likes of Warren Buffett are not immune.

In 1966, Buffet paid $4m for 5% of film studio Disney, then sold a year later for $6m. That 50% profit looked good at the time. Now it looks like a very costly mistake Disney is worth around $170bn, and that's before you consider any dividends paid out.

So how do you decide whether or not to sell? I like to keep things simple, so I'd argue that there are just two reasons to sell a share (other than the fact that you have achieved your financial goal, and now want to spend the money). Both of these, however, depend on having a sensible buying process in place.

When you buy a company, you should record your reasoning in a spreadsheet or a journal, so that you always have that to refer to (the mind will play tricks on you otherwise). And your overall portfolio of individual shares should hold a spread of companies across a range of different business sectors. As long as that's the case, then roughly 20 shares give or take will give you the benefits of diversification without overly diluting performance.

So why should you sell? Firstly, sell if there is a fundamental deterioration in the prospects of the business. If something happens (signs of fraud, say, or a major profit warning) to disprove your investment case which you wrote down when you bought the share, remember? then act. If you can't honestly say that you'd still buy today, given what you know now, then you should sell your holding entirely, and find a better opportunity. This will help you to think about what to do in situations where things have gone badly wrong.

But what of situations where everything is going right, and your main concern is that the share is getting too expensive, and you might give away profits if you hold for too long? The problem at this stage is that you might end up selling what turns out to be a Disney, rather than a momentum-driven flash in the pan.

So I'd rely on that old faithful, rebalancing. If a specific holding goes up so much that it accounts for more than a certain percentage of your portfolio, take profits to reduce its allocation. You will miss out on future potential profits, but it will also shield you from regret-driven errors (either caused by holding on too long, and so seeing your paper profits evaporate; or selling too soon, thus missing out on potential gains).

I wish I knew what a current-account deficit was, but I'm too embarrassed to ask

A country's current account is an overall measure of whether that country is a net lender to the rest of the world (the account is in deficit), or a net borrower (the account is in surplus). The current account consists of three elements. First, there's the trade balance: the total value of exports (of both goods and services) minus imports. Second is net transfers, such as remittances sent home by migrant workers, or international development spending. The third is net investment income the difference between the money the country earns on overseas investments and what it pays out to investors overseas.

To make up any difference, the country will have to sell assets; encourage foreign investment; or borrow more.In theory, if a country is running an expanding deficit, demand for its currency will fall (and thus so will its value), making imports more expensive and exports cheaper, which should improve the trade balance over time and thus narrow or close the deficit.

Deficits really only become a problem when the rest of the world loses faith in the ability of a country to fund the balance. This can lead to much-needed foreign investment fleeing the country, which in turn worsens the deficit and causes the currency to weaken. This in turn pushes up inflation weakening the economy and makes it harder to repay any foreign-currency-denominated debt. Both developed and emerging-market nations regularly run current-account deficits (the UK has run a deficit for many decades now).

But emerging markets partly due to their more fragile institutions, and partly due to the fact that more "hot" (speculative) money tends to flow their way in the good times tend to be far more vulnerable to rapid losses of confidence. This is what is happening right now in Turkey, and is also threatening to spread to other emerging nations with large current-account deficits.