How many shares should you have in your portfolio?

The more shares you have in your portfolio, the less damage any single disastrous investment will do to your wealth. But how many is too many? Matthew Partridge explains.

It's not often you get important investment lessons from popular TV programmes.

So it was nice to see that ITV managed to squeeze a warning to investors into one of the plot lines on Downton Abbey. The well-heeled Grantham family learned just how dangerous it is to put all your eggs in one basket, when they lost all their money betting on a single Canadian railway company.

However, while diversification seems to be simple common sense, just how much of it do you need? And can you have too much?

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How diversification works

diversification and asset allocation

Investing is risky. No investment is truly risk-free'. But risk comes in different forms.

For example, if you buy a share in Tesco, you are betting on the quality of the company itself. If the company runs into trouble, the shares will probably fall. So you are taking company' risk.

But you are also betting on the supermarket business. If supermarkets as a sector start to fall out of favour if the internet destroys their margins, say then Tesco is likely to suffer too. This is sector' or industry' risk.

Taking an even wider view, there's market' risk. Maybe investors just don't want to buy British-listed stocks, or shun equities altogether.

Diversification can help combat all of these risks. By buying more than one supermarket company, you could reduce company risk. By buying a business in a different industry too technology maybe you could reduce your sector risk.

And you could reduce market risk by investing some of your money in stocks listed in another country, or even in a different asset class, such as bonds.

How well does it work?

It found that the average standard deviation of a US share over a year was 49.3%. That sounds technical. What it basically means is that roughly a third of the time, you could expect the average US share to be 50% higher or 50% lower than the point at which you bought it. Very nice if you're up 50% - not so nice if you're down that much.

But if you bought another share, the standard deviation of the portfolio would fall to 37.4%. Havingten shares would cut this to 24%. Diversifying across asset classes, and across countries also helps.

How much is too much?

Unless you have a very large amount of money invested, that's going to be very expensive as a percentage of your portfolio. In other words, holding that many stocks is effectively impossible for a retail investor.

So what's the optimum amount? The good news is that Elton and Gruber found that moving from a 20 to a 1,000-share portfolio would only cut risk from 21.7% to 19.2%. So you don't need to have a ridiculously large portfolio to be properly diversified.

Many of the best investors run fairly tight portfolios. Hedge fund manager Seth Klarman holds only 22 shares in his US portfolio. He thinks the ideal number is even lower at just 15. An Australian study in 2006 also found that households with the most focused portfolios tend to do better than those who spread themselves thinner.

This makes sense. The fact is, investing is hard work. No one with a 100-stock portfolio can possibly know every one of those companies well enough to be making properly informed decisions about them.

The best level of diversification does depend partly on the sector you plan to invest in. Someone buying into a specialist, high-risk sector where lots of companies go bust, such as biotech or small miners, will want to buy enough shares to ensure their winners more than offset their many losers. This is where a fund can come in very useful.

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Dr Matthew Partridge

Matthew graduated from the University of Durham in 2004; he then gained an MSc, followed by a PhD at the London School of Economics.

He has previously written for a wide range of publications, including the Guardian and the Economist, and also helped to run a newsletter on terrorism. He has spent time at Lehman Brothers, Citigroup and the consultancy Lombard Street Research.

Matthew is the author of Superinvestors: Lessons from the greatest investors in history, published by Harriman House, which has been translated into several languages. His second book, Investing Explained: The Accessible Guide to Building an Investment Portfolio, is published by Kogan Page.

As senior writer, he writes the shares and politics & economics pages, as well as weekly Blowing It and Great Frauds in History columns He also writes a fortnightly reviews page and trading tips, as well as regular cover stories and multi-page investment focus features.

Follow Matthew on Twitter: @DrMatthewPartri