We have some exciting news to share! The Motley Fool UK has now become The Twelfth Magpie -- an independent, UK-owned company, led by our long-serving UK management team — Mark Rogers, Chris Nials and Heather Adlington. In practical terms, it’s the same team you know, now fully focused on serving our UK readers and members.

Just as importantly, our approach remains unchanged: long-term, jargon-free, and on your side. This site is our new home, and there will be extra tweaks made across the coming few days as we settle in. So if anything looks a little off, please bear with us!

The content of this article was relevant at the time of publishing. Circumstances change continuously and caution should therefore be exercised when relying upon any content contained within this article.

3 big mistakes Stocks and Shares ISA investors make

By avoiding these three simple mistakes, investors can potentially generate higher long-term returns in their Stocks and Shares ISAs.

Young Asian woman with head in hands at her desk

Image source: Getty Images

You’re reading a free article with opinions that may differ from The Twelfth Magpie’s Premium Investing Services. Become a member today to get instant access to our top analyst recommendations, in-depth research, investing resources, and more. Learn more, and get a free 'Best Buy Now' stock!.

Investing within a Stocks and Shares ISA is one of the best ways to build wealth in the UK. Sadly though, not all investors have success with their ISAs. That’s because they make investing mistakes that bring down their returns dramatically.

Here, I’m going to highlight three very common mistakes ISA investors make. Avoid these, and building long-term wealth becomes a lot easier.

Should you buy Rolls Royce shares today?

Before you decide, please take a moment to review this report first. Despite ongoing uncertainties from US tariffs to global conflicts, Mark Rogers and his team believe many UK shares still trade at substantial discounts, offering savvy investors plenty of potential opportunities to learn about.

That’s why this could be an ideal time to secure this valuable research – Mark’s analysts have scoured the markets to reveal 5 of his favourite long-term ‘Buys’. Please, don’t make any big decisions before seeing them.

Lack of diversification

Quite often, investors only own a handful of stocks. This really isn’t enough – if one or two of these stocks underperforms, they will drag the whole portfolio down.

To be fully diversified and minimise stock-specific risk, as portfolio really needs to hold at least 20 different stocks. This way, a couple of bad picks won’t have a huge impact on overall performance.

It’s worth noting here that there are plenty of ways to diversify easily today. For example, adding funds or investment trusts to a portfolio gains instant access to a broad range of shares.

Home bias

Now, a good portfolio will be diversified by asset class and within asset class. It will also be diversified geographically.

This last bit is where a lot of investors trip up. Quite often, investors experience what’s known as ‘home bias’ and invest predominantly in shares listed in their home country. This can limit their long-term returns.

Just look at the recent performance of the UK’s FTSE 100 index versus the US’s S&P 500.

Source: Google Finance

For the five-year period to the end of May, the Footsie produced a return of just 3.2% a year (including dividends) while the S&P 500 returned approximately 11% per year.

This means UK investors could have potentially improved their returns significantly by allocating some capital to US shares.

Why has the US market performed so much better than the UK market? It’s mainly because the US is home to tech giants such as Apple, Microsoft, and Alphabet – which are growing rapidly as the world becomes more digitalised.

Portfolio construction

Finally, a third mistake ISA investors make is not ‘right-sizing’ their portfolio positions. This means weighting a stock or security relative to its risk/reward profile.

Quite often, retail investors have huge positions in risky stocks (e.g. 50% of their portfolio in Tesla). This isn’t sensible from a risk-management perspective.

But neither is weighting stocks equally. Because with an equally-weighted portfolio, higher-risk stocks have the same weightings as lower-risk stocks.

Generally speaking, the best approach, when it comes to building a portfolio, is to give the most weight to solid, blue-chip stocks that have attractive risk/reward profiles and less weight to more speculative shares that could potentially blow up.

This is what professional fund managers tend to do. They often also cap position sizes at 5% to ensure that no single stock is a massive part of their portfolio.

By right-sizing their stock positions, risk can be reduced dramatically.

Generating strong returns

By focusing on these three areas of portfolio management, ISA investors can significantly reduce their overall risk levels. By doing this, they can give themselves a much better chance of generating strong long-term returns.

Ed Sheldon has positions in Alphabet, Apple, and Microsoft. The Motley Fool UK has recommended Alphabet, Apple, Microsoft, and Tesla. Suzanne Frey, an executive at Alphabet, is a member of The Motley Fool’s board of directors. Views expressed on the companies mentioned in this article are those of the writer and therefore may differ from the official recommendations we make in our subscription services such as Share Advisor, Hidden Winners and Pro. Here at The Motley Fool we believe that considering a diverse range of insights makes us better investors.

More on Investing Articles

Investing Articles

Could now be the time to buy great UK shares at bargain prices?

Some UK shares have been trading exuberantly, with the FTSE 100 hitting hew highs in 2026. Does that mean there…

Read more »

piggy bank, searching with binoculars
Investing Articles

Prediction: this stock could surge 51% in my SIPP and ISA by 2027

Ben McPoland explains why he's bullish on this growth stock in his ISA and SIPP portfolios, despite it falling 25%…

Read more »

Satellite on planet background
Investing Articles

Is SpaceX on my list of shares to buy in July?

SpaceX shares have been falling. But the wait for a return from the business might be longer than the wait…

Read more »

ISA coins
Investing Articles

£10,000 put in a Cash ISA at the start of 2026 is now worth…

We're only halfway through the year, but has a Cash ISA beaten stock market returns so far? Our writer digs…

Read more »

Young woman carrying bottle of Energise Sport to the gym
Investing Articles

Still stubbornly in pennies, will the JD Sports share price hit £1 again?

Christopher Ruane reckons the JD Sports share price looks cheap but it's already been in pennies for many months. What's…

Read more »

Middle aged businesswoman using laptop while working from home
Investing Articles

Can an ISA outperform the stock market? Yes – here’s how!

Many investors dream of using their ISA to do better than the market overall. This writer knows it's possible --…

Read more »

Asian man looking concerned while studying paperwork at his desk in an office
Investing Articles

Dear SpaceX stock fans, mark your calendar for 7 July

SpaceX stock is getting fast-tracked into the world's leading technology index. Should I buy shares of the rocket maker before…

Read more »

A young black man makes the symbol of a peace sign with two fingers
Investing Articles

Here are 2 FTSE shares I’m excited about this July — and 1 I’m avoiding

As we head into the second half of the year, Mark Hartley identifies two undervalued FTSE shares that are flashing…

Read more »