Investors don't need millions of pounds, access to hedge funds or years of knowledge and expensive qualifications to make more money. Many could greatly increase their returns simply by buying more shares. Holding too few shares is the single biggest mistake most investors make. The one factor that differentiated older, richer and more sophisticated investors was that they increased returns by holding more shares, a behavioural investment study by Goetzmann and Kumar of over 60,000 US stock holding households found. The data showed over 60 per cent of households held three shares or fewer, greatly increasing their risks.